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ELEMENTS 



OF 



STATIC ELECTRICITY 



FULL DESCRIPTION OF THE HOLTZ AND TOPLER MACHINES 
AND THEIR MODE OF OPERATING. 



/ 

By PHILIP ATKINSON, A.M., Ph.D. 




NEW YORK: 
W. J. JOHNSTON, PUBLISHER, 



168-177 POTTER BUILDING. 
1887. 



M 



Copyright, 1886, 
By W. J. Johnston. 



>LS 



INTRODUCTION. 



In this treatise the principles of electricity are 
presented untrammeled, as far as possible, by mathe- 
matical formulas, so as to meet the requirements of a 
large class who have not the time or opportunity to 
master the intricacies of formulae, which are usually so 
perplexing to all but expert mathematicians. 

This class includes those whose knowledge of 
electricity is limited to the practical details of teleg- 
raphy, telephony, and electric lighting; also those 
among the liberally educated, who desire to review 
electric science in the light of recent investigation; 
and those who wish to study its elementary principles, 
preparatory to a more extended course, which shall 
embrace all the details of electric measurement and 
electric engineering. 

The original plan included dynamic as well as static 
electricity, embracing its practical application to the 
arts; but it was subsequently thought best to confine 
the present work to static electricity alone, to meet 
the wants of those who are less familiar with its prin- 
ciples than with those of dynamic electricity, and to 



IV 1XTR0D UCTIOX. 

reserve the consideration of the latter for a separate 
volume. 

Care has been taken to avoid the introduction of 
new matter before the student was prepared for it ; 
hence it was thought best that there should be a 
thorough examination of elementary principles before 
introducing complicated apparatus, the construction 
and operation of which depends on those principles. 

The theory assumed is, that electricity is one of 
the forms in which energy manifests itself; that its 
nature is not changed by the means emplo} r ed to 
generate it, and that the various terms, positive, 
negative, static, dynamic, express certain conditions and 
relations in which this manifestation occurs, and 
not different kinds of electricity. 

The author takes pleasure in acknowledging his 
obligations to Elisha Gray for the use of tables, giving 
the results of observations on earth currents, made 
under his direction on the Postal Telegraph line; also 
to Ferguson, Gordon, Silvanus P. Thompson, Noad 
and Deschanel, from whose excellent works valuable 
assistance has been obtained, though he has felt 
compelled to dissent from some of their views. 

The views here expressed are the result of many 
years' experience in the class room, the lecture room, 
and the laboratory, and were adopted only after the 
most rigid test of actual and oft repeated experiment. 
And some of the more important apparatus described 



INTRODUCTION. V 

is of the author's own manufacture, constructed in 
strict accordance with electric principles, verified by 
his own experiments. 

While humbly following the great pioneers in 
electric science, who have hewed waymarks on the 
rocks, the author will rest content if he has left some 
foot-prints on the sands, which may serve to guide the 
wayfarer till obliterated by the coming waves of 
progress. 

The impartial criticism of teachers and electricians 
is especially requested, that faults and errors may be 
corrected in future editions. 

PHILIP ATKINSON. 

Chicago, June, 1886. 



CONTENTS. 



CHAPTER I. 

Page 

Attraction and Repulsion, 1 

Conductors and Non-Conductors, ... 4 

Quantity and Intensity, 6 

Static Electricity Defined, 8 

CHAPTER IT. 
Electric Potential, 10 

CHAPTER HI. 
The Nature of Electricity, 23 

CHAPTER IV. 
Induction, 43 

CHAPTER V. 
Electric Distribution and Condensation, . . 55 

CHAPTER VI. 

Accumulators, 72 



Vlll CONTENTS. 

CHAPTER VII. 
Electric Generators. — 

The Electrophorus and Frictional Machine, 92 



CHAPTER VIII. 
Electric Generators. — 

The Holtz and Topler Machines, . . . los 

CHAPTER IX. 
Experiments with the Topler Machine, . . . 125 

CHAPTER X. 
Electric Transmission in Vacua, .... 14(5 

CHAPTER XI. 
Electrometers, . 155 

CHAPTER XII. 
The Electricity of the Earth and Atmosphere. — 

Potential and Earth Currents, . . . 175 

CHAPTER XIII. 
The Electricity of the Earth and Atmosphere. — 

The Aurora, 190 

CHAPTER XIV. 
The Electricity of the Earth and Atmosphere.— 

Lightning and Thunder, ..... 207 



ELEMENTS 

OF 

STATIC ELECTRICITY. 



CHAPTER 



Attraction and Repulsion — Conductors and 
Non-Conductors — Quantity and Intensity — 
Static Electricity Defined. 

Attraction and Repulsion. — Amber, called in 
Greek tjtextQov, was known to the ancients to acquire, 
when rubbed, the power of attracting light bodies ; 
hence this property, now known to belong to all sub- 
stances, has received the name of electricity. The 
earliest conception of electricity, then, was that of 
force and the latest discoveries sustain this view. 

Electricity may be generated by various simple 
methods, as follows:— Let a spoon be balanced on the 
edge of a cup. and an ebonite (hard rubber) knife- 
handle, rubbed on a woolen or silk fabric, be held 
near it. and the spoon will be attracted. Substitute for 
the knife-handle a stick of sealing-wax. a lamp-chim- 
ney, or a paraffin wax-candle, rubbed in the same way. 
and the spoon will be attracted by each of them. 

These different substances may be multiplied, and 
different rubbers used, but it will be found that the 



Z ELEMENTS OF STATIC ELECTRICITY. 

attractive force, though variable in intensity, is com- 
mon to all. 

The balanced rod, represented in Fig. 1, will be 
found more convenient for these experiments than the 
balanced spoon. It consists of a round wooden rod, 
about twenty inches long, and half an inch in diameter, 
with the ends rounded or terminating in balls. It is 
pivoted at the center on a point, and may be mounted 
on a stand, or on a bottle with a pin through the cork, 
and made to revolve rapidly by the force of attrac- 
tion, following any of the electrified bodies already 
mentioned when held near it, as represented. 




Fig. 1— The Balanced Rod. 

A more sensitiveinstrument for investigations of this 
class is represented in Fig. 2, and known as the pith- 
ball electroscope; the name electroscope being derived 
from the Greek axonem, to see, i]lr/.rnor, electricity. It 
is constructed as follows.: A small brass rod, bent at 
right angles, has its short arm inserted into an ebonite 
stem attached to a wooden base, giving the instrument 
a vertical height of about Hi inches. The horizontal 
arm is about 8 inches long, and terminates in a small 
brass ball. From this arm two pith balls, each about 
half an inch in diameter, are suspended by silk threads. 



ATTRACTION AND REPULSION. 



Let the pith balls be separated at the points of sus- 
pension, so that when they hang vertically a consider- 
able space shall intervene between them, and let a 
stick of sealing-wax, previously electrified by friction, 
be brought near one of them ; the ball will be attracted 
to the wax, and, after a momentary contact, repelled. 
Follow it with the wax, and it continues to recede as if 
pushed back by some invisible barrier. 

Now let the other pith 
ball be moved near this 
one, and they will be 
attracted to each other, 
and, after contact, re- 
pelled : the lines of sus- 
pension showing diver- 
gence in each direction 
as represented. 

Let the electrified wax 
be again brought near, 
and each ball is repelled 
by it, so that when it is Fig ' 2 ~ Tbe Pitll " Ba11 E1 ^rosco P e. 
placed between them, they are driven further apart; 
but let any non-electrified body be brought near and 
they are attracted to it. 

If each of the balls be separately electrified by the 
wax, and they are then brought near each other, they 
will show mutual repulsion without previous attraction. 

From this series of phenomena we learn, first, that 
electrified bodies not only attract non-electrified bodies, 
as already shown, but communicate electricity to them 
by contact ; and, secondly, that bodies electrified, either 
by each other or from the same source, show mutual 
repulsion. 




4 ELEMENTS OF STATIC ELECTRICITY. 

The first fact was shown when the pith ball, after 
contact with the wax, attracted and electrified the 
other pith ball; and the second fact by the repulsion 
of the pith ball from the wax after contact; then of 
the two pith balls from each other and from the wax, 
after contact ; and finally by the mutual repulsion of the 
balls, without previous attraction, after being separately 
electrified by the wax. 

This series of phenomena may be produced by using 
a glass or ebonite rod, or anj^ of the substances already 
mentioned, as well as by the sealing-wax; showing that 
repulsion as well as attraction is a property common to 
all electrified bodies. 

Conductors and Xox-Coxductors. — Pursuing our 
investigation, new properties are developed. It is 
found that while certain substances, as glass, ebonite, 
and sealing-wax, show electric qualities, others, as brass, 
iron, and copper, apparently do not show such qualities. 
This led to the old division of all substances into 
electrics, a term applied to the former, and non-electrics, 
applied to the latter. 

But more thorough investigation has proved that 
electricity may be generated by friction on the brass, 
iron, and copper, as well as on the glass, ebonite, and 
sealing-wax ; but that, when generated on bodies of the 
former class, it is instantly distributed over the entire 
body, and escapes to the earth unnoticed, if the body 
be held in the hand, while, when generated on bodies 
of the latter class, it is not so distributed, and does not 
pass off in this way; bodies of the former class allowing 
free electric movement, over the surface or through the 
mass, while those of the latter class resist such move- 
ment. 




CONDUCTORS AND NON-CONDUCTORS, 5 

To make this evident, let a short brass rod, of about 
quarter inch diameter, terminating in a ball, be fitted to 
an ebonite handle, as 
represented by Fig. 3. 
Let the brass be rubbed 
briskly on woolen, silk, Fi - Z~ ThQ Insulated Metal Rod. 
or India rubber, holding the instrument by the handle, 
audit will attract and repel the pith balls in the same 
way as the other electrified substances already used. 
Copper, iron, or any other metal may be substituted 
for the brass with the same result. 

Repeat the experiment, allowing the metal to touch 
the hand, and the electric qualities disappear. This 
shows that in the first experiment the electricity was 
retained, because it could not pass through the ebonite 
handle ; while, in the second, it passed off through the 
hand. 

The results obtained by experiments of this kind led 
to the abandonment of the doctrine of electrics and 
non-electrics, and the classification of all bodies as con- 
ductors or non-conductors. 

Experiment proves that all substances conduct elec- 
tricity, and that they all offer a certain amount of 
resistance to its passage. But it is found that the 
relative proportions of conductivity and resistance vary 
greatly in different substances. In some the conduct- 
ivity is largely in excess, and they are called conductors ; 
in others the resistance is largely in excess, and they 
are called non-conductors. Between these extremes 
there are all degrees of variation; so that in some sub- 
stances the two properties are almost equally balanced. 
Hence, since no exact rules can be given, we distinguish 
the two classes by saying that a CONDUCTOB i* any suh- 



b ELEMENTS OF STATIC ELECTRICITY. 

stance of such low resistance that it can be used practi- 
cally for the transfer 'of electricity ; and a NON-CON- 
DUCTOR is any substance of such high resistance that it 
can be used practically to prevent such transfer. 

List of Conductors and Non-Conductors. — The 
principal conductors are the metals, silver and copper 
being the best. Among the partial conductors are the 
different varieties of carbon, including coal, charcoal, 
and graphite ; the acids, saline solutions, water, vegeta- 
bles, and animals. 

The principal non-conductors are caoutchouc, gutta- 
percha, sulphur, and their compound, known as hard 
rubber, vulcanite, or ebonite ; dry air, paraffin, shellac, 
amber, resin, glass when free from metallic substances, 
mica, silk, fur, wool, hair, feathers, bisulphide of carbon, 
petroleum, and oil of turpentine. 

Among the partial non-conductors are porcelain, 
baked wood, paper, and leather. 

Insulator Defixed. — When a non-conductor is 
used in connection with a conductor to confine elec- 
tricity within certain limits, it is called an insulator; 
and the conductor on which the electricity is confined, 
or to be confined, is said to be insulated ; as a metal 
placed on a glass or ebonite support, a copper wire 
wrapped with silk or wool. 

Quantity and Intensity. — Electric quantity and 
intensity are similar to the quantity and intensity found 
in other more familiar forms of energy. The intensity 
of any form of energy, other things being equal, is in- 
versely proportional to the mass of the body in which it 
is developed. A few strokes of a hammer on a small 
piece of iron placed on an anvil will raise its temper- 
ature to a burning heat; while the same number of 



QUANTITY AND INTENSITY. 7 

strokes on a large mass of iron will produce but very- 
slight change of temperature. The quantity of muscular 
energy expended is the same in each case, but the inten- 
sity of heat energy produced varies inversely as the 
mass. 

The intensity varies also as the resistance. A small 
piece of wood held in the flame of a lamp is quickly 
ignited at the end in the flame ; while the end held in 
the hand shows no perceptible change of temperature. 
But a brass rod of the same size, similarly held for the 
same length of time, becomes too hot for the hand long 
before the end in the flame is hot enough to ignite 
wood. 

In the wood, the intensity rises rapidly at the end 
held in the flame, because the resistance prevents dis- 
tribution of heat through the mass. But the low 
resistance of the brass permits the rapid distribution 
of the same quantity of heat through its mass ; so that the 
intensity at the end in the flame is much less than that 
of the wood. 

In kindling a fire of anthracite coal, when the pro- 
portion of coal is too great for the kindling-wood, the 
heat generated by the consumption of the wood fails to 
ignite the coal, because such coal being a comparatively 
good conductor of heat, the amount is rapidly distrib- 
uted through the mass, and hence the intensity at any 
point is insufficient to produce ignition. But if the pro- 
portion of coal be sufficiently reduced, the consumption 
of the same amount of wood will produce ignition. 
The quantity of heat imparted to 1 lie coal is the same in 
each case, but its intensity is greater in the latter case. 

In electric experiments there is a great difference 
noticeable in the amount of work required to produce 



8 ELEMENTS OF STATIC ELECTRICITY. 

the same electric intensity on different bodies. Sealing- 
wax and ebonite, for instance, are quickly electrified, 
while brass is electrified slowly. The reason is analo- 
gous to that in the illustrations just given : the brass 
being a good electric conductor, the electricity is in- 
stantly distributed equally over every part of its sur- 
face, and hence the quantity at any point being small, 
the intensity is low. But the sealing-wax and ebonite 
being good non-conductors, the same quantity of elec- 
tricity is concentrated on those parts of the surface 
brought into immediate contact with the rubber, instead 
of being equally distributed over the entire surface ; 
and hence the intensity at those points is proportion- 
ately increased. 

It will be shown hereafter that in static electricity 
the electrification is on the surface. Hence, in this 
case, electric intensity means quantity in proportion to 
surface, whether it be the entire surface, as on a con- 
ductor, or only those parts to which the electrification 
is confined, as on a non-conductor. 

It must also be understood, as will be shown more 
fully hereafter, that the term intensity is as applicable 
to a diminution of electric energy at a given point as 
to an increase ; in the same sense as we speak of intense 
cold, as well as of intense heat. 

Static Electricity Defined.— The terms used to 
distinguish different classes of electric phenomena, as 
frictionah static, galvanic, chemical, magneto, thermo, 
take their origin from the different methods by which 
electricity is generated, and the various conditions under 
which its phenomena have been observed, and should 
not be understood as referring to any difference in 
the nature of the electricity produced. 



STATIC ELECTRICITY DEFINED. 9 

The term frictional has been used to designate that 
class of phenomena now under consideration, since 
friction is one of the principal agencies by which the 
electricity is generated. But it seems more appropriate 
to use a term embracing, not merely one agency by 
which the electricity is generated, bat also the various 
phenomena produced, and distinguishing these phenom- 
ena from those pertaining to electricity generated by 
other methods. And since these phenomena refer 
chiefly to electricity when stationary, the term static, 
from the Latin sto, to stand, has been adopted, to dis- 
tinguish electricity observed under these conditions 
from electricity observed chiefly in a state of motion. 



CHAPTER II. 
Electric P otenti al . 

Potential. — Potential, in the physical sense, is the 
power to accomplish work. It derives its specific name 
from the nature of the work, as gravity potential, heat 
potential, electric potential. 

A pound weight raised to the height of ten feet has 
acquired ten foot-pounds of gravity potential, and has 
the power, if allowed to descend to the same level, of 
accomplishing ten foot-pounds of work, either in rais- 
ing another weight, or setting machinery in motion by 
which work may be accomplished. 

A mass of metal whose temperature has been raised 
from zero to one thousand degrees, has acquired one 
thousand degrees of heat potential, and can accomplish 
work to that amount in cooling to zero, either by heat- 
ing another mass, or generating steam by which machin- 
ery can be put in motion and work accomplished. 

We have seen that bodies, when electrified, acquire 
the power to attract or repel other bodies. This power 
is called electric potential. 

Suppose that the electric energy of the sealing-wax 
in attracting the balanced rod, represented in Fig. 1, 
Chapter I., w^ere just sufficient, if expended without 
loss, to move the rod one foot ; and, in doing so, to 
overcome a resistance from inertia and friction repre- 
sented by two ounces (one-eighth of a pound ) ; the 



ELECTRIC POTENTIAL. 11 

electric potential of the sealing-wax would equal one- 
eighth of a foot-pound. 

If only half this energy were required to overcome 
inertia and friction, the other half might be expended in 
lifting to a height of one foot an ounce weight attached 
to a thread fastened to the end of the rod, and passing 
over a pulle}'. In which case the work accomplished 
by this half would be represented by one-sixteenth of 
a foot-pound. Or the weight might be raised, or other 
work to the same amount accomplished, by putting in 
motion light machinery connected with the rod by gear- 
ing at its center ; the added friction being included in 
the ounce representing friction and inertia. 

Impulsion would evidently produce the same results 
in this case as attraction. 

To distinguish between electricity and electric poten- 
tial, we must bear in mind that electricity represents 
the energy itself, while potential represents certain rela- 
tions between this energy and matter. Hence we derive 
the following definition : 

Electric potential is the power which a body possesses to 
accomplish work by virtue of its electricity. 

Difference of Potential. — To accomplish work 
in tliis way there must first be a difference of po- 
tential. 

The descending weight could not raise the other 
weight unless there was a difference of level between 
them. Tin,' heated metal could not heat a similar mass 
unless there was a difference of temperature between 
them. Neither could the electrified sealing-wax attract 
the rod unless there was a difference of electric energy 
between them. And these phrases, difference of level. 
difference of temperature, difference of electric energy, 



12 ELEMENTS OF STATIC ELECTRICITY. 

are simply different forms of expression for difference of 
potential. 

To produce this difference work must first be ex- 
pended, and this work is the measure of the potential 
acquired. 

The lifting of the pound weight ten feet against the 
force of gravity gave it the ten foot-pounds of gravity 
potential. The work of heating the metal, whether 
represented by combustion, by friction, or by concus- 
sion, gave it the one thousand degrees of heat potential. 
And the rubbing of the sealing-wax gave it the one- 
sixteenth of a foot-pound of electric potential. 

As there is ordinarily no practical difference of elec- 
tric potential between different points on the earth, 
within a limited area, its potential is considered zero, 
and taken as the base of all measurements of electric 
potential. 

The qualification of this statement, as above, becomes 
necessary, since there are often great differences of po- 
tential over widely separated areas. 

Positive and Negative. — Bodies whose potential 
is higher than that of the earth are said to have positive 
potential, while those whose potential is lower are said 
to have negative. 

The potential of bodies is also considered positive or 
negative with reference to each other. If a body has 
a higher potential than the earth, but lower than that 
of another body, it is said to be positive with reference 
to the earth, but negative with reference to the other 
body. In like manner a body may have negative 
potential with reference to the earth, but positive with 
reference to another body of lower potential. 

Hence, positive and negative are merely convenient rela- 



ELECTRIC POTENTIAL. 13 

tive terms to designate different degrees of potential and 
not different kinds of electricity. 

The sign ( + ) is used to denote positive potential, and 
( — ) to denote negative potential. * 

The earth's potential, then, is the electric zero, just 
as the freezing point is the zero of temperature in the 
centigrade thermometer, and all uninsulated bodies are 
said to be connected ivitli the earth, and to have zero 
potential when not under special influence from insu- 
lated, electrified bodies in their vicinity. 

When the electric potential of a body is changed 
from zero by an increase of its electricity, it is said to 
be positively electrified ; and when its potential is 
changed from zero by a decrease, it is said to be nega- 
tively electrified. 

Electric Movement. — When a difference of electric 
potential exists between different bodies, or different 
parts of the same body, there is a constant tendency to 
equalization. 

A state of equilibrium seems to be the natural condi- 
tion of bodies, and to produce difference of potential 
requires, as we have seen, the exercise of force in the 
performance of work, by which this equilibrium is dis- 
turbed. 

We find in other forms of energy, as gravity and heat, 
the same tendency to equilibrium, requiring the exercise 
of force to overcome it, as in the illustrations already 
given. 

The restoration of equilibrium is always effected by 
a transfer of energy from the body having the greater 
to the one having the less energy; that is, from higher 
to lower potential. 

In the case of gravity this transfer of energy carries 



14 ELEMENTS OF STATIC ELECTRICITY. 

the body with it, as in the descent of a weight or the 
movement of water from a higher to a lower level. 
But in the case of heat and electricity, the energy may 
move while the body remains stationary ; and it may be 
transferred from one body to another, or from one part 
to another of the same body. Thus the mass of metal, 
in the illustration given, transfers its heat energy to 
another mass ; and in like manner, when a metal rod is 
heated at one end, the heat moves to the cold end. 

Gravity apparently can move only by carrying the 
body with it, while heat moves through the body with- 
out producing change of position in its mass, like gravity. 

A hot body transfers its heat to a cold one in its 
vicinity, but does not attract it ; while gravity produces 
mutual attraction between all bodies, but is not trans- 
ferred like heat from one body to another. 

But in electrified bodies we have both kinds of move- 
ment. Like heat, electricity can move from one body 
to another, or from one part to another of the same 
bod}' ; and, like gravity, it can cany the body with it. 

Hence we must distinguish between the movement of 
electricity and the movement of the electrified body. 
Electric movement, like heat movement, is from higher 
to lower potential. If one part of a conductor be elec- 
trified, the electricity instantly distributes itself over 
every part. If two insulated bodies, free to move, are 
placed in each other's vicinity, like the pith balls of the 
electroscope, the same tendency to equilibrium is shown 
by their mutual attraction. 

Though only one ball be electrified, yet it is evident 
that their movement toward each other must be mutual, 
and in proportion to their mass, since action and reac- 
tion are equal : so that while the movement of the elec- 



ELECTRIC POTENTIAL. 15 

tricity is from the electrified to the non-electrified ball, 
that is, from higher to lower potential, the movement 
of the balls is mutual. 

It will also be noticed that the movement of the non- 
electrified ball is opposite to that of the electricity. 
Hence, while electricity moves from higher to lower 
potential, bodies under its influence may move in -either 
direction. 

We have seen that when the two balls come into 
contact there is a transfer of electricity from the elec- 
trified to the non-electrified ball ; equilibrium is estab- 
lished, and mutual repulsion follows, not only between 
the balls, but also between them and the electrified 
sealing-wax. 

So long as a difference of potential exists there is 
mutual attraction; but when equilibrium is established 
there is mutual repulsion. The same results may be 
produced by numerous similar experiments, in which 
different substances and different methods may be 
employed. Hence we deduce the following important 
principle : • 

Electrified bodies at different potentials attract, tvhile 
those at the same potential repel each other. 

There can be no repulsion unless there is a difference 
of potential between the electrified bodies and their 
surroundings. For if the surrounding bodies were at 
the same potential as the electrified bodies, the repul- 
sion would be neutralized by their reaction. Hence 
bodies at zero potential can show no repulsion. But in 
all cases of electrification there is a difference of poten- 
tial created in the body, either above or below the origi- 
nal zero. 

Indeed, attraction may account for the apparent 



16 



ELEMEXTS OF STATIC ELECTRICITY. 



mutual repulsion of bodies at the same potential, since 
this difference of potential between the electrified bod- 
ies and their surroundings must produce attraction and 
tend to separate them. 

But such outward attraction would not disprove the 
existence of repulsion, though it might account for 
some of its phenomena. 

The Gold Leaf Electroscope — As our investi- 
gations now require a more sensitive instrument than 
any which has yet been described, we here introduce 
the gold leaf electroscope. 




-3 

1 



Fig. 4— Gol-1 Leaf Electroscopes. 



The style represented at A, Fig. 4, is convenient, and 
easily constructed. It consists of a half-gallon tincture 
bottle, fitted with an ebonite stopper, through the center 
of which passes a small brass rod about five inches long, 
which terminates about three-fourths of an inch above 
the stopper in a brass disc about two inches in diame- 
ter, having a round rim about three-sixteenths of an 



ELECTRIC POTENTIAL. 17 

inch in diameter projecting from its lower surface, as 
shown in the enlarged section at D. 

To the lower end of the rod is attached a thin cross- 
bar, about five-eighths of an inch long, which will pass 
easily through the neck of the bottle. And from this 
cross-bar are suspended two strips of imitation gold leaf, 
each five-eighths of an inch wide by 2 J- inches long. A 
small hole is drilled near the edge of the disc for con- 
venience in attaching wires. 

The leaves in this instrument lie close together, and, 
consequently, must always be electrified at the same 
potential; but in some experiments it is desirable to 
electrify them separately, and for this purpose a bottle 
with a wide neck is used, which will admit an ebonite 
stopper through which two rods can be inserted about 
an inch apart, and from the cross-bar of each a single 
leaf is suspended, the surfaces being parallel to each 
other. This style is represented at B, Fig. -i. The rods 
can terminate above in balls, or be bent outward and 
terminate in discs. 

Electroscopes may be constructed with thin metal 
discs, attached to the glass opposite the leaves ; strips 
of the same material extending down and connecting 
with the earth. Brass rods surmounted with balls are 
often used in the same way, as represented at C\ Fig. 4 ; 
in which case a glass shade resting on a wooden base is 
more convenient than the bottle form. 

The object in either case is to have conductors at zero 
potential near the leaves, which renders them more 
sensitive, and discharges them in case of too great 
divergence ; thus preventing their adhesion to the glass, 
which is often troublesome. Annoyance from the latter 
cause is also obviated by using a bottle of globular form, 



18 ELEMENTS OF STATIC ELECTRICITY. 

the sides of which are too remote to be touched by the 
leaves. 

A brass cap, covering the glass above, as shown at (7, 
is also used to screen the leaves from external electric 
influence, and wire screens are likewise used for the 
same purpose. 

The use of the bottle, or glass shade, is to protect the 
leaves from currents of air which would destroy them. 
And the ebonite stopper is for better insulation, since 
the glass generally used for bottles and shades is of 
inferior insulating quality. The disc, or ball, and con- 
necting-rod are for convenience in electrifjdng the 
leaves, which are the efficient part of the instrument. 

The following experiment will illustrate its use: — 
Let the electrified sealing-wax touch the disc of electro- 
scope A ; electricity is instantly transferred to the disc, 
rod, and leaves, which are all good conductors, and the 
leaves, being free to move, and at the same potential, 
repel each other, and diverge. 

If the disc now be touched with the finger, the elec- 
tricity escapes to the earth, and the leaves, being reduced 
to zero, converge. 

The sensitiveness of this instrument is so great that a 
chip of dry wood, less than a grain in weight, electrified 
in cutting, and dropped on the disc, produces divergence 
of the leaves. A wooden pen-holder, electrified by strik- 
ing it lightly on the table, produces the same effect. 

Hence, care must be observed to prevent the leaves 
from being torn by sudden, spasmodic movements, which 
are liable to occur when experimenting with highly elec- 
trified bodies in their vicinity. 

Mutual Effects of Friction.— Thus far we have 
considered only the effect produced on the sealing-wax, 



ELECTRIC POTENTIAL. 19 

glass, or other substance electrified by friction, without 
reference to the effect on the substance by which it was 
rubbed. But since action and reaction are equal, it is 
evident that these two effects must, in some way, equal 
each other ; that electricity, or its equivalent in some 
other form of energy, must be produced on the rubber 
as well as on the substance rubbed. 

To test this, let a piece of flannel, after being used to 
rub a stick of sealing-wax, touch the disc of electroscope 
A, Fig. 4, and the leaves will instantly diverge, showing 
that the flannel has been electrified. 

Substitute silk, fur, or any other substance used as a 
rubber, and the same result will follow. Let the various 
substances rubbed be also tested, and it will be found 
that electrification has been produced on both rubber 
and substance rubbed, at the same time, by the same 
process. 

Now let a rubber, about the same size as the sealing- 
wax, be prepared, by wrapping a strip of wood in flannel 
and insulating one end with a piece of india-rubber 
tube. 

Holding this rubber by the insulated end, let the 
sealing-wax be rubbed with it ; and, keeping both still 
in contact, lay them carefully on the disc of the electro- 
scope, so that both shall touch it at the same instant, 
and no divergence of the leaves will occur. Now lift 
off the sealing-wax and they instantly diverge; replace 
it and they converge. Lift off the rubber and they 
diverge, replace it and they converge again. 

Let the experiment be made with any other two sub- 
stances used to generate electricity by friction, as silk 
and glass, ebonite and fur, and similar results will be 
obtained. 



20 ELEMENTS OF STATIC ELECTRICITY. 

It will also be noticed that the approach of either 
electrified body while the other lies on the disc causes 
the leaves to converge, while its withdrawal produces 
divergence. 

There is often a slight divergence of the leaves when 
both bodies are in contact on the disc, due to the diffi- 
culty of producing perfect adjustment of contact, and 
also to the fact that the electric condition of one body 
may change more rapidly than that of the other, from 
imperfect insulation or other cause. 

The amount of divergence is also liable to vary, the 
removal of one body producing greater divergence than 
the removal of the other. This difference is also easily 
accounted for by difference of mass, of conductivity, or 
other cause. 

Hence we deduce the following rule : When electricity 
is generated on two bodies by their mutual friction, the elec- 
tricity of each is neutralized by the presence of the other. 

The effect of the mutual friction of the two bodies is 
to create a difference of potential by the transfer of 
electric energy from one to the other. As one gains 
what the other loses, the amount of energy on the two 
is not changed so long as they remain in contact, and 
hence the potential of the electroscope is not disturbed. 

But let one of the bodies be removed ; suppose it to 
be the one to which energy has been transferred, the 
potential of the remaining body being negative, there is 
instantly a transfer of energy to it from the disc and 
leaves, which thus become negative also. 

The leaves, being both at the same potential, diverge 
by mutual repulsion ; and that potential being less than 
zero, the divergence is increased by attraction from the 
higher potential of the glass and surrounding objects. 



ELECTRIC POTENTIAL. 21 

Replacing this body, let the one from which energy 
has been transferred be removed ; the potential of the 
remaining body being positive, there is instantly a 
transfer of energy from it to the disc and leaves, making 
them positive also. Hence the leaves diverge as before, 
from mutual repulsion, and the divergence is increased 
by attraction from the loiver potential of the glass and 
surrounding objects. 

From this it will be seen that the effect on the electro- 
scope is the same whether the potential of the electrified 
body be positive or negative. In either case there is 
mutual repulsion between the leaves, from their being 
at the same potential ; and mutual attraction between 
them and surrounding objects, caused by difference of 
potential. 

The indications of the electroscope furnish no means 
of distinguishing between positive and negative poten- 
tial, being the same for both. And as this is true of 
most of the phenomena pertaining to these two states, 
it is difficult, in static electricity especially, to determine 
which phenomena are positive and which negative. 

There is no such well-marked distinction between 
them as between the positive and negative states known 
as heat and cold; neither can we observe electric move- 
ment as we can heat movement; since heat moves 
slowly, while electricity moves with inconceivable ra- 
pidity. 

But if we can show cause for an accumulation of elec- 
tric energy at one point and Tor its absence at another, 
and show effects following such difference of energy, we 
then have proof of the positive and negative potential 
of the different points, which may be accepted as 
reliable. 



22 ELEMENTS OF STATIC ELECTRICITY. 

Such proof will be furnished hereafter, and the further 
consideration of this question must be deferred till the 
examination of other phenomena shall enable the stu- 
dent to comprehend such proof. 

Charge Defined. — The term charge is used to ex- 
press the condition of an electrified body when its poten- 
tial is above or below zero. If its potential has been 
raised above zero by receiving electricity, it is said to be 
positively charged ; but if its potential has been reduced 
below zero by the removal of electricity, it is said to be 
negatively charged. 

Hence we speak of a high negative charge in the same 
sense as we speak of intense cold, meaning an intensity 
of the negative condition caused by the absence of heat. 



CHAPTER III. 
The Nature of Electricity. 

The Conservation of Energy. — A clear under- 
standing of that great doctrine of modern science, 
known as the conservation of energy, lies at the founda- 
tion of a correct knowledge of electricity and electric 
phenomena. Hence a brief examination of its prin- 
ciples will not be out of place here. 

Energy is a universal property of matter. It is the 
principle of life and movement in matter in distinc- 
tion from matter itself, inseparably connected with 
matter and yet distinct from it : heat as distinct from 
the heated body ; electricity as distinct from the elec- 
trified body ; life as distinct from the living body. 

Like matter, it manifests itself in various forms, 
as gravity, cohesion, chemical affinity, light, heat, elec- 
tricity. Like matter, its quantity in the universe is 
fixed and definite, and cannot be increased or dimin- 
ished. And hence, like matter, it is indestructible. 

It maybe transmuted- from one form into another, 
but in the transmutation there is no loss. One form 
may re-appear in many forms, or the many be reduced 
to the one. 

In our experiments, muscular energy lias been ex- 
pended to produce electric energy; but the energy pro- 
duced must equal that which produced it, if the doc- 
trine of the conservation of energy is true. And since 



24 ELEMENTS OF STATIC ELECTRICITY. 

it is evident that only a very small part of the mus- 
cular energy expended would be required to move 
the pith balls, the balanced rod, or the gold leaves, the 
remainder must be accounted for. 

This is easily done when we consider, first, that 
the electric energy was equally divided between the 
rubber and the substance rubbed; secondly, that only 
a small part of the electric energy was used : that the 
electricit}^ generated was sufficient for the performance 
of the same work many times in succession, either 
with the rubber or substance rubbed; and that a 
number of pith balls, placed on all sides of the 
electrified body, might have been subjected to its 
influence. Thirdly, we must consider the amount of 
electricity lost from contact with the surrounding air: 
and, lastly, that the amount of heat energy gener- 
ated by the friction was probably equal to the electric 
energy. 

If the expended energy had been produced by a 
descending weight, which should cause a glass or 
ebonite cylinder to revolve in contact with a rubber, 
and the sum total of the heat and electricity had 
been recovered in the form of work which could be es- 
timated, it would be found so nearly equal to the 
number of foot-pounds expended by the descending 
weight, that whatever difference existed could easily 
be accounted for by the friction of the machinery and 
other causes. 

Experiments of this kind have been actually per- 
formed, and the results verify the above conclusion. 
Similar experiments have also been made with other 
forms of energy, and like results obtained; so that 
the principles of the conservation of energy are now 



THE NATURE OF ELECTRICITY. 25 

well established, and universally recognized in all 
practical work. 

Another illustration may render the subject more 
clear. A pound weight raised to a height of 20 feet 
has acquired 20 foot-pounds of energy ; and, in de- 
scending to its former level, can accomplish 20 foot- 
pounds of work; as in raising to the same height 
another weight of nearly equal mass. But if stopped 
in its descent at a level of 10 feet, it has expended 
only 10 foot-pounds of energy, and has still a reserve of 
10 more. 

It is evident) that to raise the weight in the first place 
required the expenditure of 20 foot-pounds of energy ; 
and that though this energy was consumed in the pro- 

>s, and had disappeared, it was not lost, but merely 
stored up, ready to be expended, either at once, by 
the weight descending the entire distance, or in detail, 
as when stopped half way or at any other point. If it 
had descended but one foot, it would still have a re- 
serve of 19 foot-pounds of energy. 

It will be noticed that the second weight, raised by 
the descent of the first, is required to be of less 
magnitude, since part of the energy must be expended 
in overcoming friction and inertia. For* if it were of 
equal magnitude, the force expended would exceed the 
force stored up; Bince it must perform not only the 
same work-, but the added amount for friction and 
inertia; in which case it would be possible to create 
force, and the doctrine of the conservation of energy 
would cease to be true. 

It is immaterial whether the descent of the one pound 
raises a small weight to the height of 20 feet, or a 
large weight to the height of one foot; which it can be 



20 ELEMENTS OF STATIC ELECTRICITY. 

made to do by a system of ropes and pulleys. The 
20 foot-pounds of energy expended must exactly 
equal the 20 foot-pounds stored up; and the height 
through which the large weight is raised is to that 
through which the small weight descends in the in- 
verse ratio of the mass of each. 

In all electric work, of whatever nature, the same 
principle will be found to hold true ; gravity potential 
in this case representing electric potential in electric 
work. Mechanical work may re-appear as electric work, 
or electric as mechanical work; the energy produced 
being always, in some form, equal to the energy 
expended. 

Heat, Light, and Electricity Compared. — The 
weight of evidence goes to show that electricity, like 
heat and light, belongs to that kind of energy known 
as molecular ; and whatever is known as to one kind of 
energy may, by analogy, be inferred as to other kinds 
of the same class, with such modifications as distin- 
guish different species of the same genus. 

There is ample proof that heat is a mode of molec- 
ular motion. Not that the heat produces the motion, or 
the motion the heat, but that it is motion ; that the 
molecules of matter being thrown into a certain kind 
of motion, the result is the sensation known as heat. 

According to the universally accepted theory of 
light, it is another species of motion of the same 
kind; and there are indications that light and elec- 
tricity are identical. But, if not identical, we may 
at least assume that they are closely allied to each 
other. 

We find also that the same causes, acting at the 
same time, on the same bodies, and under the same 



THE NATURE OF ELECTRICITY. 27 

circumstances, produce both heat and electricity, in 
numerous instances; in others, equally numerous, both 
heat and light, and in others, heat, light, and elec- 
tricity. 

The simultaneous production of heat and electric- 
ity is seen in the examples already given of bodies 
electrified by friction, of which heat is also a neces- 
sary result. 

Another prominent instance is the action of the 
electric generators known as dynamos ; in which the 
evolution of heat is such that special provision for cool- 
ing has to be made, to prevent injury. Here mechani- 
cal action is the accent. 

In the galvanic batten* we have a well-known in- 
stance of the production of heat and electricity by 
chemical action ; as a certain amount of heat, more or 
less perceptible, is always a result. 

Instances of the simultaneous production of heat 
and light are numerous and well known, as the heat- 
ing of an iron rod, which becomes luminous when the 
temperature rises to a certain degree ; whether it be 
heated by friction, as of a shaft and journal, or by the 
chemical action of a furnace. 

The dynamo and galvanic battery have been re- 
ferred to as producing both heat and electricity. 
When a current of this electricity, of sufficient in- 
tensity, is passed through a conductor of high resist- 
ance, as a fine platinum wire, or a carbon filament, they 
become luminous by incandescence; and when passed 
through two sticks of carbon slightly separated, we have 
light of great intensity : and there is, in both instances, 
the evolution of intense heat. These are perhaps the 
most striking examples which can be given of the sim- 



28 ELEMENTS OF STATIC ELECTRICITY. 

ultaneous evolution of heat, light, and electricity from 
the same causes. 

In the thermo-electric battery we have an example 
of the direct production of electricity by heat. 

Polarized Light axd Electkiclty. — Experiments 
with polarized light, made by Faraday and others, fur- 
nish strong evidence of the close alliance, if not actual 
identity, of light and electricity. 

It is known that ]ight, from certain peculiarities of 
reflection and transmission, undergoes a change, so 
that when subsequently transmitted, its action is dif- 
ferent from that of the original transmission, and this 
change has been termed polarization. 

Let a plate of tourmaline be so placed that a ray of 
light falling on it shall be transmitted at right angles 
to a particular direction within the crystal, known as 
its optical axis. Then let another tourmaline plate be 
so placed with reference to this one that their optical 
axes are parallel, and that the ray shall pass through 
both and form a bright spot on a screen beyond. 

Now let either plate be turned, so that their optical 
axes are no longer parallel to each other, but still at 
right angles to the ray ; the bright spot on the screen 
will fade as the angle increases, till at 90 degrees it will 
disappear. Continuing the rotation, it will re-appear, 
increasing in brightness, till, at 180 degrees, it is en- 
tirely restored ; then fading again till extinguished 
at the end of the third quadrant, and again increasing 
in brightness till restored at the end of the fourth 
quadrant, or original position. 

This alternation of brightness and extinction de- 
pends on the relative angular position of the plates. 
Let either of them be turned in either direction, and 



THE NATURE OF ELECTRICITY. 29 

the same result follows ; but when both are turned in 
the same direction, there is no change in the brightness, 
and no extinction of the light. Substitute one for the 
other, and the same results are obtained. 

It is evident, then, that the light in passing through the 
first plate has undergone a change which affects its trans- 
mission through the second, in any position except when 
the optical axes of both are parallel; extinguishing it 
entireh T when they are at right angles to each other. 

It is also found that this effect, termed polarization, 
occurs to light transmitted through or reflected from any 
transparent medium, as glass, selenite, Iceland spar, 
and various liquids. Polished metals also produce the 
same effect on reflected light. And this reflection or 
transmission takes place at a certain angle, known as 
the polarizing angle, which varies in each substance 
by a certain definite amount. 

Now let the ray be transmitted through, or reflected 
from, a small piece of glass of suitable size or shape, 
placed at the proper polarizing angle, and let the plates 
be turned till their optical axes are at right angles, so 
as to produce extinction ; then let the glass be sub- 
jected to a powerful electric strain and the extinguished 
light will re-appear, continue during the electric action. 
and disappear when it ceases; which shows that this 
electric action has counteracted the effects of polar- 
ization. 

Similar experiments with various substances, too 
numerous to detail here, show similar result-. 

Without anticipating another branch of our subject, 
it may 1)0 stated here, that electricity and magnetism 
are so closely allied that whatever affects one must 
have some important relation to the ether. 



30 ELEMENTS OF STATIC ELECTRICITY. 

Recent experiments have demonstrated that unusual 
disturbances in the variation of the magnetic needle 
are coincident with unusual disturbances in the sun, in 
connection with the phenomena known as sun spots ; 
and that the telegraph and telephone are, at such 
times, seriously disturbed by what are known, techni- 
cally, as " electric storms"; that is, unusual disturb- 
ances in the earth's electricity shown in the phenomena 
known as earth currents. 

It has also been found that these solar disturbances 
are periodic; and that these periods, for the last hundred 
and fifty years, correspond almost exactly with the 
periods of unusual variation of the magnetic needle. 

Since the sun is our chief source of light and heat, 
since they are, in fact, the result of the constant dis- 
turbance of the elements of that body; and since, when 
this disturbance assumes an unusual phase, there is, co- 
incident with it, an unusual disturbance in the earth's 
electricity, it must be accepted as strong proof of a 
common origin of heat, light, and electricity. 

We have heat, light, and electricity derived from 
friction, from chemical action, from magnetic action, 
and from the sun. We have heat producing electric- 
ity, and electricity producing heat and light ; and we 
have electricity neutralizing the polarizing effect of 
light. The evidence of identity then becomes cumu- 
lative, while that of close alliance amounts to demon- 
stration. Hence we may infer certain facts in regard 
to the nature of electricity from what we know of 
similar facts in regard to the nature of light and heat. 
And we are also warranted in the conclusion, that a 
well supported theory of light or heat requires but 
little modification to adapt it to electricity. 



THE NATURE OF ELECTRICITY. 31 

This much we certainly know, that they all are forms 
of energy and that they radiate from the centers where 
they are generated. Hence the term radiant energy 
is equally applicable to each. 

The Wave Theory. — It has been assumed that a 
subtle medium, termed ether, pervades all space ; that 
it is so attenuated that it can insinuate itself between 
the grosser molecules of material bodies; so that solids 
of the finest and closest texture, as well as liquids and 
gases, are pervaded by it; and that light, and probably 
electricity, are due to waves or undulations of this 
ether. The evidence of the existence of such a medium 
is almost wholly negative, and, like all negative evi- 
dence, unsatisfactory. The assumption presupposes the 
necessity of its existence. 

It has been stated that energy is a universal property 
of matter, and the converse may be accepted, that 
energy cannot exist without matter. And since light, 
coming from the sun, must traverse the interplane- 
tary spaces, there must be matter there ; else we shall 
be compelled to admit that energy can exist without 
matter, which is contrary to all our experience. 

The earth is surrounded by an atmosphere, to the 
limits of which we cannot penetrate. In 1822, Dr. 
Wollaston made a careful mathematical calculation, as 
the result of which lie claimed to have demonstrated 
that the earth's atmosphere must have limits, beyond 
which it cannot exist. And this apparent demonstra- 
tion was accepted as authority, and remained unchal- 
lenged for half a century. Meantime the w;ive theory 
oi light became prominent, and a medium being one of 
its fundamental principles, the existence of the ether 
was assumed, and is now generally accepted. 



82 ELEMENTS OF STATIC ELECTRICITY. 

But the researches of modern science have swept 
away many of the errors of the past, and it is not im- 
possible that Dr. Wollaston's demonstration may share 
the same fate. Many eminent scientists, who have 
made experimental investigations on the subject, hold 
that the expansibility of the earth's atmosphere is 
unlimited ; among whom may be cited Grove, Gassiot, 
Geissler, and Dr. Andrews. And W. M. Williams, 
in his work, " The Fuel of the Sun," claims to have 
discovered a serious error in Dr. Wollaston's calcula- 
tions, which vitiates his conclusion. 

The assumption of these writers is that an atmos- 
phere, the same as that of our earth, pervades all 
space ; that in the interplanetary spaces it becomes ex- 
ceedingly attenuated ; and that each of the heavenly 
bodies attracts and surrounds itself with a portion of 
it; the extent and density of which is in proportion to 
the mass of the body. 

The high degree of vacuum which can now be attained 
by improvements in the air-pump, seems to demonstrate, 
that while electricity will pass more freely through rare- 
fied air, on account of the reduced resistance, than 
through air of ordinary density, it must still have a 
medium in which to travel ; and that its passage through 
an absolute vacuum, or space devoid of any known ma- 
terial substance, is highly improbable. But as the best 
attainable vacuum is still only an approximation to an 
absolute vacuum, the full demonstration of this point 
has not yet been reached. 

The existence then of some elastic medium, by which 
the two forms of radiant energy, known as light and 
heat, can traverse the interplanetary spaces, is not 
questioned. Nor does the theory of the unlimited ex- 



THE NATURE OF ELECTRICITY. 33 

pansion of our atmosphere conflict at all with the 
theory of the universal existence of ether, since 
the theory of ether is that it permeates all material 
substances. 

The wave theory assumes that radiant energy is 
transmitted by the undulations of some medium ; that 
an impulse originating at any center of energy, as the 
sun, produces a wave which traverses this medium with 
inconceivable velocity, till it reaches some distant 
point, as the earth ; and that the constancy of such im- 
pulses at every point on the sun gives rise to the phe- 
nomena of solar light, heat, and electricity. 

In like manner we may assume any other center of 
energy, as a red-hot metal ball, radiating light and 
heat; a stick of ebonite, excited by friction, radiaiing 
electricity. 

It is also assumed that the impulses radiate in straight 
lines, while the undulations occur at right angles to 
those lines. 

To illustrate : — Drop a pebble on a smooth sheet of 
water; the impulse creates waves which radiate out- 
ward in widening circles. The pebble has depressed 
the water at the point where it struck, forcing the ad- 
jacent water outward, and causing it to rise above the 
general level; then the downward impulse of this 
wave, linking under the force of gravity, raises the 
original center, and also produces, by ks outward im- 
pulse, another wave beyond, as it descends by its inertia 
below the general level. 

As the water oscillates vertically above and below the 
level, each successive? impulse produces a new wave, 
while the same process goes on in the outward waves, 
creating new waves beyond, in ever widening circles, 



34 ELEMENTS OF STATIC ELECTRICITY. 

till the force of the original impulse has been exhausted, 
and the water returns to its original level. 

Now it will be perceived that there is no transfer of 
the water from the center outward bej^ond the length of 
the first wave. Part of the water forced outward by the 
original impulse flows back again, while another part 
flows outward, producing a new wave. The water is then 
under the influence of two forces, one horizontal, the 
other vertical, acting at right angles to each other; the 
horizontal producing the wave length, that is, the 
distance from crest to crest, or from hollow to hollow, 
while the vertical produces the height, that is, the 
vertical distance from hollow to crest, or from crest to 
hollow. 

In a similar way, it is supposed, occur the undula- 
tions of the assumed elastic medium, with this excep- 
tion, that the waves on the water occur in the same 
horizontal plane, radiating outward in concentric cir- 
cles, while those in the elastic medium occur in any 
direction in which they are free to move ; radiating 
outward in concentric spheres, if wholly unrestrained , 
or in sections of spheres or spheroids, if limited and 
starting from impulses at various points on any surface, 
either spherical, like that of the sun, or plane. 

Having taken an illustration from a liquid, illus- 
trations from solids will also be in point. 

If a long rope, stretched lengthwise, with plenty 
of slack, be held at one end, and jerked rapidly up and 
down, it will be thrown into waves, which will run 
along its entire length. 

Here it is evident that while the impulses given at 
one end travel in waves to the other, the rope, as a 
whole, remains stationary; successive portions acting 



THE NATURE OF ELECTRICITY. 35 

as yielding levers to transmit the impulse along its 
length. 

If one end of a lever be depressed below a horizontal, 
it receives a forward as well as downward movement, 
in the arc of a circle, its opposite end receiving an up- 
ward and backward movement. In this way each suc- 
cessive portion of the rope oscillates horizontally as 
well as vertically, modified by the difference between a 
yielding and a rigid body. 

Let a number of elastic balls be suspended in a 
straight line in contact with each other. Draw back 
the outer ball at one end of the line and let it swing 
against the adjoining ball ; the impulse will be trans- 
mitted along the line, and the outer ball, at the other 
end, will swing off to nearly the same distance as that 
through which the first ball swung, all the others re- 
maining stationary. 

Here the impulse is transferred from ball to ball by 
virtue of their elasticity. When number one impinges 
on number two the impact changes its shape slightly to 
that of a spheroid; as it resumes its shape it imparts the 
impulse to number two, by which it is imparted to num- 
ber three, and so on through the line. But action and 
reaction being equal and opposite, there is no perceptible 
movement till the last ball is reached, which swings off, 
since there is no ball to react against it. The impulse 
travels, but the line remains stationary. 

Here, also, it will be perceived that there is a radial 
force acting at right angles to the horizontal force, 
much the same as would result from a similar impact 
if each ball were hollow, and its surface composed of an 
infinite number of semicircles, joined at the points of 
impact. 



86 ELEMENTS OF STATIC ELECTRICITY. 

Now the molecules of a metal rod may be com- 
pared to an infinite number of these lines of balls; and 
it may be assumed that a beat impulse, or an electric 
impulse, given at one end, moves along these lines 
in some way analogous to that in which the impulse 
moves along the lines of balls. 

We are not obliged to confine ourselves to any spe- 
cific method of movement; but may suppose a wave 
movement, similar to that which takes place in the 
slack rope, or on the water, if it shall seem to accord 
best with known facts and phenomena. 

In the polarization of light, it is supposed that the 
waves assume a certain phase, in conformity with the 
special arrangement of the molecules of the crystal. 
Hence if the crystals are cut from the same block, and 
placed in the same position, the phase will be the same 
for each, and the light will pass through. But if the 
second is turned at right angles to the first, the phase 
produced by passing through the first will not be in con- 
formity with the arrangement of the molecules in the 
second, and hence the light cannot pass through. 

Suppose the arrangement of the molecules to be in 
layers, or strata, like the sheets composing a ream of 
note-paper, placed in a vertical position; the waves 
of ether would assume a vertical phase, and, meeting 
the second crystal, placed in the same position, would 
pass through. But if the second were turned, so as to 
bring its strata to a horizontal position, the vertical 
waves would be broken, and could not pass through. 

Instead of the ether we may suppose the molecules 
themselves thrown into waves, whose phase would con- 
form to the structure of the crystal, and the same result 
would evidently follow. 



THE NATURE OF ELECTRICITY, 37 

There is no reference, in this supposed case, to any 
visible stratification of a crystal, as the experimental 
ray is usually admitted at right angles to such stratifi- 
cation ; the reference is to an invisible arrangement of 
the molecules. 

The same course of reasoning is applicable to heat, 
or to electricity, but the phase of the heat wave, or 
the electric wave, may be different from that of the 
wave of light, so that a substance opaque to light, as 
copper, might allow the free passage of heat or electric- 
ity ; or a substance transparent to light, as glass, 
might obstruct their passage. 

And the medium in which the electric energy travels 
may be the ether which is supposed to pervade the 
different kinds of matter; or the matter itself, in any 
of its various forms, solid, liquid, or gaseous: as it 
has been shown that undulations may take place in 
each of them. 

Conductivity for Heat and Electricity Com- 
pared. — It is very remarkable, and must be something 
more than mere coincidence, that conductivity for heat 
and electricity is nearly the same in the same sub- 
stances. A good heat conductor is a good electric 
conductor; a non-conductor of heat is a non-conductor 
of electricity. So that if we know either the conduc- 
tivity or resistance of any substance for heat, we 
have, ] ractically, its conductivity or its resistance for 
electricity. This will appear from the table follow- 
ing, showing ihe results obtained by Wiedmann and 
Franz. 

Hence if heat and light are modes of motion, travers- 
ing various substances by undulations, we arc warranted 
in assuming the same with reference to electricity. 



38 elements of static electricity. 

Comparative Conductivity of different sub- 
stances FOR HEAT AND ELECTRICITY, AS GIVEN BY 
WlEDMANN AND FRANZ : — 

Substance. Heat Conductivity. Electric Conductivity. 

Silver 100 100 

Copper 74 73 

Gold 53 59 

Brass 24 22 

Tin 15 23 

Iron 12 13 

Lead 9 11 

Platinum .... 8 10 

German silver . . 6 6 

Bismuth .... 2 2 

Other observers place the electric conductivity of 
some of these substances much higher, making the con- 
ductivity of copper nearly equal to that of silver. 

If the electric wave has its own peculiar structure, it 
is evident that a substance whose structure is adapted 
to it, or whose molecules easily adapt themselves to it, 
would be a conductor; while a substance whose struct- 
ure is not so adapted, or whose molecules resist such 
adaptation, would be a non-conductor. 

An attempt to insert a No. 36 screw into a No. 30 
screw hole will fail, because the threads of the screws 
are not adapted to each other. But let the same screw 
be inserted into some yielding substance, as soft wood, 
and the substance adapts itself to the structure of the 
screw ; or, as we say, it cuts its own thread ; while a 
rigid substance like iron resists such adaptation. 

Something analogous to this may constitute the 
difference between conductors and non-conductors, 
and may also be the cause of other electric phenomena 
of equal importance. 



THE NATURE OF ELECTRICITY. 39 

The Spark and Sxap. — As already stated, every 
substance offers a certain degree of resistance to the 
passage of electricity, and a result of this resistance 
is the generation of heat, often accompanied with light, 
and this effect varies as the resistance. 

Platinum is a metal of high resistance, while that of 
copper is very low; and a fine platinum wire will be 
brought to a white heat by an electric current which 
would scarcely change the temperature of a copper wire 
of the same size. 

Air offers such high resistance that the passage of 
electricity through it always produces both heat and 
light, in the form of a bright spark. This occurs not 
only when an electric charge passes through several 
inches of it, but through the thinnest film ; the pres- 
ence of heat, as well as light, being demonstrated by the 
lighting of gas by a spark less than i inch in length. 

A sudden condensation of the air, forced forward 
and laterally by the charge, has been suggested as 
the probable cause. If such condensation takes place, 
heat would certainly be the result, as when air is com- 
pressed by mechanical means. And perhaps it would 
be accompanied by light, though this is not probable, 
as combustion and incandescence are the only known 
means of producing artificial light besides that now 
under consideration, either of which would imply the 
presence of some other substance besides air. But the 
hypothesis seems to assume the passage of some material 
substance through the air to produce the condensation, 
of which there is no evidence. 

But if, instead of condensation, we suppose undula- 
tions to take place, giving greal intensity of motion. 
as the electric impulse, darting forward with incon- 



40 ELEMENTS OF STATIC ELECTRICITY. 

ceivable velocity, suddenly forces the resisting air into 
the phases of the electric waves ; then the generation 
of those other modes of motion, known as heat and 
light, might easily be the result. 

A sharp sound, varying from an insignificant snap to 
a deafening report, always accompanies the spark. On 
the condensation theory this is accounted for by the 
sudden displacement and reflux of the air. But since 
sound, like heat and light, is another mode of motion, 
occurring chiefly in the air. it is evident that the wave 
theory will best account for it ; the electric impulse 
giving rise to these different modes of motion. 

The Dual Theory. — We have already seen, in ex- 
periments with the pith ball electroscope, that the balls 
may be attracted and repelled by electrified glass, seal- 
ing-wax, and various other substances. 

Let an electrified glass rod approach one of the balls ; 
the ball is attracted, and, after contact, repelled. Now 
let an electrified stick of sealing-wax be brought near, 
and the electrified ball, which was repelled by the glass, 
is attracted by the wax. Or let the ball be first elec- 
trified and repelled by the wax, and it is attracted by 
the glass. 

Further experiments show that the same results can 
be produced with other substances; glass representing 
a certain class of substances, which show similar electri- 
fication, and sealing-wax and resin another class, which 
shows opposite electrification to that of glass. 

Hence it has been assumed that there are two kinds of 
electricity. One kind generated on the class of sub- 
stances represented by glass, and the other on the class 
represented by resin. The former was once designated 
as vitreous, and the latter as resinous; but the term 



THE NATURE OF ELECTRICITY. 41 

positive is now used instead of vitreous, and negative 
instead of resinous. Used in this way, these terms 
have no reference to a difference either in quantity or 
intensity ; they express only a supposed difference in 
kind, not in degree. 

This doctrine of the dual nature of electricity was 
first proposed by Dufaye, and has since been strongly 
maintained by many eminent scientists. Deschanel, 
speaking of the phenomena under consideration, says : 
" These phenomena clearly show that the electricity de- 
veloped on the resin is not of the same kind as the elec- 
tricity developed on the glass." 

Now the only thing "clearly shown " is the difference 
in the substances, not in the electricity. For we have 
precisely the same electric phenomena of attraction 
and repulsion with the glass as with the sealing-wax ; 
but a third substance, the electrified pith ball, is at- 
tracted by one and repelled by the other; a result 
which it would seem more reasonable to attribute to 
the difference knoivn to exist between the substances, 
than to a difference supposed to exist in the electricity. 
For it has already been shown that different causes, as 
conductivity or resistance, influence the intensity of 
electrification on different substances. Other causes 
also, as a difference of temperature or mass, of hard- 
ness or softness, density or porosity, doubtless contribute 
to the same result. 

But considering the quality of resistance alone, the 
potential of any non-conductor, as glass, is liable to 
vary greatly on different parts of its surface, when 
electrified by friction ; and to differ from the potential 
of sealing-wax, similarly produced on different parts 
of its surface. 



42 ELEMENTS OF STATIC ELECTRICITY. 

The friction of the same rubber is also greater on 
a substance like sealing-wax, whose surface is soon soft- 
ened by the heat generated, than on a smooth, hard sub- 
stance like glass, which is not affected in this way. 

And, as already shown, difference of potential produces 
attraction, while equality of potential produces repulsion, 
between bodies. Hence the attraction by the sealing- 
wax, of the pith ball electrified by the glass, is a neces- 
sary result of difference of potential ; while its repul- 
sion by the glass follows from equality of potential. 
And the same will be true of the ball electrified and re- 
pelled by the wax and attracted by the glass. 

But if a difference exists in the kind of electricity 
produced by the different classes of substances, Ave 
should expect that difference always to manifest itself 
whenever one of either class is employed as a generator. 

But this is only true in a general way, to which the 
exceptions are very numerous ; for it often happens that 
glass and sealing-wax, or other substances belonging to 
the different classes, when rubbed with the same rubber, 
exhibit the same electric qualities. The same result 
also will often follow where different kinds of rubbers 
are employed, as silk on one substance and woolen on 
the other. 

Such results are inconsistent with the theory of two 
electricities ; but are easily accounted for by a differ- 
ence, or an equality of potential, which we know is 
liable to exist. 

Hence, preference must be given to the doctrine of one 
electricity, originally proposed by Franklin ; simple and 
plain, like truth itself, and in strict accord with all elec- 
tric phenomena; whether pertaining to static electric- 
ity, or to electricity under other forms. 



CHAPTER IV. 
Induction. 

It is noticeable that in all our experiments thus far 
the electrified body acts on the other bodies before there 
is any actual contact. The knife-handle attracts the 
spoon ; the sealing-wax, ebonite, or glass attracts the 
balanced rod or the pith ball while separated from them. 
And when either of these electrified bodies approaches 
the gold-leaf electroscope, there is first a divergence of 
the leaves before contact occurs. 

It will also be noticed that this effect increases or 
diminishes as the distance is increased or diminished. 
And, further, that while the interposition of different 
substances, as glass, paraffin, ebonite, air, wood, metal, 
produce great variations in the effect, none of them 
wholly prevent it. 

There is evidently, then, an invisible influence ex- 
tending to a certain distance from the electrified body 
in every direction, and affecting everything within its 
sphere, and this effect is called induction 

When an electrified body is brought near the disc of 
the electroscope without touching it. the leaves diverge, 
and on its removal converge again, showing no perma- 
nent effect. But if it is allowed to touch the disc, the 
leaves are electrified, and remain divergent after its 
removal. 

But if, instead of touching the disc, it be held near 



44 



ELEMENTS OF STATIC ELECTRICITY. 



enough to produce divergence, as at J., Fig. 5, and, while 
in that position, the disc be touched with the finger, as 
at B, the leaves will converge, and remain so as long 
as the electrified body is held near ; but on its removal 
as at (7, they will diverge, and remain divergent, the 
same as after contact of the electrified body with the 
disc. 




Fig. 5 — Induction Illustrated. 



Here, then, is electrification by induction, without 
any transfer of electricity by contact. How can this be 
accounted for ? 

When the electrified body is brought near, whether 
its charge be positive or negative, the effect of induction 
is to produce a temporary change of the potential of 
the electroscope, and the leaves diverge. 

If the charge of the electrified body be positive, elec- 
tricity is repelled from the disc to the leaves, and they 
diverge, being positively electrified to the same poten- 
tial, and hence mutually repellent, and also attracted 
by the lower potential of surrounding bodies. 

But if the electrified body be negatively charged, 



INDUCTION. 45 

electricity is attracted from the leaves to the disc, and 
they diverge, being negatively electrified, and mutually 
repellent, as before, and attracted by the higher poten- 
tial of surrounding bodies. 

Now, when the disc is touched with the linger, and 
thus connected with the earth, if the charge is positive, 
the potential of the electroscope is changed by the 
escape of electricity to the earth under the influence of 
the electrified body, and the leaves converge. But if 
the charge is negative, the potential of the electroscope 
is changed by the attraction of electricity from the earth, 
and the leaves converge as before, equilibrium being re- 
stored between the disc and leaves in each case. 

The leaves remain convergent so long as the electrified 
body is held near ; the electroscope being still under the 
influence of the force by which the change of potential 
was produced ; which is evidently just equal to the re- 
pelled energy in the first instance, and to the attracted 
energy in the second. But when the electrified body is 
removed, this equilibrium is disturbed, and the leaves 
diverge under the influence of mutual repulsion and 
outward attraction, as already explained. 

This experiment proves that a body connected with 
the earth, and under the influence of induction, may 
differ in potential from the earth, and is not necessarily 
at zero potential from its earth connection. For it is 
evident that such difference of potential existed during 
the connection of the electroscope with the earth, else 
it could not have become manifest when the connection 
was severed and the inductive influence removed. For 
when the electrified body is removed before such con- 
nection, the leaves converge, but when removed after it 
has been made and severed, they remain divergent; 



46 ELEMENTS OF STATIC ELECTRICITY. 

showing that the difference of potential was created and 
existed during the earth connection. 

This point has an important bearing on phenomena to 
be considered hereafter, in regard to which prominent 
writers have been betrayed into serious mistakes from 
having overlooked it. 

Influence of Distance. — It is important to notice, 
that when the electroscope has been charged b}~ induc- 
tion in this manner, and the electrified body is again 
brought near, the leaves continue to converge as the 
body approaches, and come together when it is in the 
same position as when the disc was touched with the 
finger. A nearer approach produces divergence, which 
increases as the body is brought still nearer. 

Let it now be gradually withdrawn, and the leaves 
gradually converge and come together, when the body 
reaches the same point, as before. Further withdrawal 
produces divergence, which continues to increase, and 
reaches its limit when the body is wholly removed. 

If the electroscope be placed at various points, equally 
distant from the electrified body, the effect of induction 
will be the same, so long as the same distance from the 
earth and surrounding objects is maintained. Hence 
it is evident that electric energy, like other forms of 
radiant energy, as light and heat, radiates equally 
in all directions when not interfered with by other 
influences. 

Suppose the electrified body to be a small globe, 
entirely removed from the earth, and surrounded with 
a perfectly homogeneous medium, it would be the 
center of a sphere of inductive influence. And suppose 
the lines of force radiating from it to be cut by the 
surfaces of two imaginary concentric spheres of differ- 



INDUCTION. 47 

ent sizes, one placed outside of the other, and having 
the electrified globe for their common center. 

Since the surfaces of spheres are to each other 
as the squares of their radii, and since the radii meas- 
ure the distances from the center, the surfaces are to 
each other as the squares of their distances from the 
center. 

But as each surface embraces all the lines of force, 
the intensity of force on equal surface areas of the two 
spheres would be in the inverse ratio of their entire sur- 
faces ; and hence would vary inversely as the squares 
of their distances from the center. 

Hence, electric induction varies inversely as the square 
of the distance. 

Practically the conditions of the supposed case are 
never exactly fulfilled ; but that does not affect the 
correctness of the principle, which is the same in elec- 
tricity as in light and radiant heat. 

Cylinder Electrified by Induction. — The effect 
of induction may be further illustrated by an insulated 
cylinder of conducting material, placed between two 
spheres of similar material, one insulated, and the other 
connected with the earth by a chain, as shown in Fig. 
6; the cylinder having mounted on it three pith-ball 
elec es, connected with it by conductors. 

It* the insulated sphere, .1, be positively electrified, 
electricity will be repelled by induction from the end of 
the cylinder next A to the end next //. And since B 
is connected with the earth, the electricity accumulated 
on the end of C\ next to it, will repel to the earth from 
B an amount equal to the positive charge? on A. 

Hence the pith ball next .1, being negative and A 
positive, is attracted by A % while the one next />, being 



48 ELEMENTS OF STATIC ELECTRICITY. 

positive and B negative, is attracted by B ; but the 
central ball, being neutral, remains unmoved. 

If the sphere, A, be negatively electrified, these condi- 
tions will all be reversed. Electricity will be attracted 
to the end of C next A, and a positive charge, equal 
to the negative on A, attracted from the earth to B. 
Hence the balls will assume the same positions as 
before. 




Fig. G— Cylinder Electrified by Induction. 

Similar inductive effects can be produced on the cyl- 
inder by the sphere A alone, but less marked than when 
two spheres are used; and, for such an experiment, tin- 
foil electroscopes are better than those made with pith 
balls, being more sensitive. 

Theory of Induction. — It Is not known how in- 
ductive force is transmitted. The hypothesis has been 
advanced that it is by a certain strain of the medium; 
as when a weight is lifted by a rope or pushed by a pole, 
the energy is transmitted in one case by the tension of 
successive portions of the rope, and in the other by a 
compression of successive portions of the pole. In 
either case the energy or stress produces a strain, which 



INDUCTION. 49 

runs through the substance of the medium till it reaches 
the object, and the continued stress produces continued 
strain. Something analogous to this, it is assumed, 
takes place in the transmission of electric energy by 
induction. 

This hypothesis has the sanction of eminent author- 
ity, and may assist us in arriving at a solution of the 
problem. Gordon says: "If electric induction were a 
6 direct action at a distance,' we should expect that it 
'would be transmitted equally through all insulators. 
One of the strongest arguments for supposing it to be 
a strain of the particles of the insulator is found in the 
fact that different insulators transmit it with very differ- 
ent strengths." 

"Induction, so far from being a 'direct action at a 
distance,' is most certainly transmitted by the particles 
of the dielectrics, and is affected by almost every molec- 
ular change which may occur in them." 

And he defines strain, as here used, to mean " an 
alteration of size or shape," including "all alterations 
of volume," "all twistings and bendings, and all vibra- 
tory motions other than those of a rigid body as a 
whole." 

The wave theory agrees with the views here expressed ; 
for we have only to conceive that this "strain" consists 
in a "vibratory motion," that is, in undulations of the 
medium. 

It is also in accordance with the analogy of similar 
transmission of other forms of radiant energy. And, if 
all energy has a common origin, it is reasonable to sup- 
pose that the transmission of its different forms would 
present striking analogies. 

Influence of Dielectric. — In order to observe 



50 ELEMENTS OF STATIC ELECTRICITY. 

induction there must be two or more bodies at different 
potentials placed in each other's vicinity, and these must 
be separated by an insulator; for, if separated by a con- 
ductor, equilibrium would at once be restored, and in- 
duction could not take place. 

Insulators through which induction takes place are 
called dielectrics, from the Greek &«, through. Air was 
the dielectric between the electroscope and electrified 
body, and between the spheres and cylinder, in the ex- 
periments already given. 

Now, since conductors permit electricity to pass 
through them easily, while insulators resist its passage, 
there must be some peculiarity in the nature or arrange- 
ment of the molecules which makes two bodies of the 
same class similar in this respect, while two of opposite 
classes are dissimilar. 

Hence we can easily conceive that when two insulated 
conductors, at different potentials, are brought into con- 
tact, the undulations of their molecules would assume 
the same phase, and equilibrium take place; but that 
when those undulations are transmitted through a die- 
lectric, they undergo such a change that, the phases of 
the undulations not being the same, there is a repulsion 
instead of an intermingling, which results in creating 
opposite potentials in adjacent parts, on either side of 
the dielectric, the negative of one being equal to the 
positive of the other. 

And since in the transmission, part of the energy is 
consumed in overcoming the resistance, difference of 
potential, on opposite sides, must result from this cause 
also. 

If either conductor be removed, still remaining insu- 
lated, the equilibrium of each will be restored, and its 



IXDUCTION. 51 

potential be found the same as before it was brought 
within the sphere of inductive influence, showing that 
no permanent effect has resulted. 

Hence it will be seen that the effect of induction is 
opposite to that of contact; the latter producing perma- 
nent equilibrium between conductors, while the former 
produces temporary disturbance of equilibrium. 

Specific Ixductive Capacity. — It has already been 
stated that electric induction takes place through all 
substances, but in different degrees ; and, since it 
is found that each has an inductive power peculiar 
to itself, this property is called its specific inductive 
capacity. 

The importance of this subject will be understood 
when it is considered that it affects enterprises in- 
volving large capital, public convenience, and public 
safety : as in the transmission of electric energy by 
insulated conductors, as telegraph and telephone wires, 
ocean cables, and electric light wires; including the 
important question of underground transmission in 
cities. 

Hence, for the last forty years, it has engaged the atten- 
tion of such men as Faradaj^, Boltzmann, and many 
others, including the earlier researches of Cavendish, 
who to have been the first to investigate it, but 

whose experiments on this subject have only recently 
been published. 

The genera] method of investigation is as follows: — 
The inductive capacity of dry air at the barometric 
pressure of 7<>0 millimeters (29.92 inches) and at the 
temperature of 0° C. (32 Fahrenheit) is made the 
standard unit by which the inductive capacities of all 
other substances are estimated. 



52 ELEMENTS OF STATIC ELECTRICITY. 

To illustrate : — Suppose we have two insulated metal 
plates, A and B (Fig. 1), separated by an air space C; 
let A be electrified and B connected with tlie disc of 
an electroscope. First note the amount of divergence 
of the leaves ; then let a plate of glass, cake of paraffin, 
or any other insulator which will exactly fill the space 




Fig. 7— Specific Inductive Capacity Illustrated. 



(7, be introduced between the plates, and note the diver- 
gence of the leaves now, as compared with the former 
divergence. 

As this insulator has displaced the air, it is evident 
that its inductive capacity, as compared with air, is 
shown by the difference in the divergence of the leaves. 



INDUCTION. 53 

If that divergence has increased, then the power of this 
insulator to transmit electric influence — that is, its spe- 
cific inductive capacity — is greater than that of air; 
otherwise, it is equal to, or less than that of air. 

From this we see that specific inductive capacity varies 
inversely as insulation. Hence this property is almost 
infinite in the best conductors ; while in the best insu- 
lators it is the reverse. 

By methods similar to the above, with the aid of 
improved instruments, to be described hereafter, the 
specific inductive capacities of a number of substances, 
including the principal insulators, have been carefully 
estimated by Boltzmann, Gordon, and others: and from 
the results obtained by them the table on the next page 
has been prepared, in which the general averages are 
given. 

The results obtained by different observers differ so 
widely that they can onl} r be regarded as approximate, 
and will undoubtedly require future correction, when 
improved methods shall give greater accuracy. 

The table shows the electric resistance of glass to be 
much less than that of ebonite ; the inverse ratio being 
5.87 to 2 89 : and this is doubtless true of glass, in the 
average. But, if the best insulating glass were com- 
pared witli the best insulating ebonite, the ratio might 
require to be reversed. Ebonite, Avhen subjected to a 
powerful electric strain, seems to yield gradually, and 
allow the electricity to creep through it ; and, by con- 
tinued strain, its electric resistance soon becomes 
permanently impaired: while the best insulating glass 
rigidly resists, and suffers fracture before yielding. 

But, according to Gordon, the electric resistance of 
glass also becomes somewhat impaired by long use ; or, 



54 ELEMENTS OF STATIC ELECTRICITY. 

which is the same thing, its specific inductive capacity 
is increased. All of which goes to prove that electric 
transmission depends on molecular structure. 



Specific Ixductive Capacities of Various 
Substances. 

Standard. 
Air at 0° C. temperature and 760 mm. pressure, 1.0 

Solids. 

Paraffin, 2.09 

Caoutchouc, 2.23 

Gutta-percha, 2.46 

Shellac, 2.85 

Ebonite, 2 89 

Sulphur, 2.95 

Resin, 8.6 

Glass, average of various kinds, 5.87 

Liquids. 

Bisulphide of carbon, 1.81 

Petroleum, 2.05 

Oil of turpentine, 2.19 







Gfases. 








Hydrogen, 


H, 


at 0° 


c. 


and 760 mm 


., .99941 


Carbonic oxide, 


CO, 


U 




u 


1.00001 


Marsh gas, 


CH 4 , 


u 




t6 


1.00035 


Carbonic dioxide, 


co 2 , 


u 




CC 


1 00036 


Nitrous oxide, 


NO, 


u 




OC 


1.00039 


Olefiant gas, 


C 2 H 4 , 


U 




" 


1.00072 



CHAPTER V. 
Electric Distribution and Condensation. 

Equipotential. — A charge of electricity given to 
any part of a good conducting surface is instantly dis- 
tributed equally over every part, and such a surface is 
called equipotential. For the momentary increase of 
electric energy at any point creates electric movement 
from higher to lower potential, which instantly results 
in the establishment of equilibrium at every point. 

Separate points on such a surface are called equipo- 
tential points, and a line of such points an equipotential 
line. 

Lines of Force. — The direction along which elec- 
tricity tends to move, from a point of higher to one of 
lower potential, is called a line of force. Such lines 
are perpendicular to the equipotential surfaces at the 
points ; for, as the tendency is to move from one point 
to the other, it would be from one such surface to the 
other; and if the line differed from a perpendicular, it 
would imply, by the resolution of forces, that there 
could be two lines of force at right angles to each 
other, one of which would lie in an equipotential sur- 
face ; implying two points at different potentials in such 
surface, which would be an impossibility. 

Surface Condensation. — Since the surface of a 

solid sphere of any good conducting material is evi- 
dently equipotential, we may regard its interior as 



56 ELEMENTS OF STATIC ELECTRICITY. 

composed of an infinite number of such surfaces, or 
spherical shells, having a common center ; and their 
radii as equipotential lines cut by such surfaces. From 
which it is evident that no difference of potential could 
exist in the interior of such a sphere. 

If it were insulated, a positive charge communicated 
to it would evidently be distributed equally through 
every part, if there were no influence tending to pro- 
duce a different effect. But, since the sphere would 
be at a higher potential than its surroundings, induc- 
tion would create lines of force in the direction of the 
radii, which must result in the condensation of the en- 
tire charge on the surface. 

Also, since every portion of the sphere is at the 
same potential, and since electrified bodies at the same 
potential repel each other, it is evident that the mole- 
cules would be self-repellent. But since they are rigid, 
the electricity of each molecule would repel that of 
every other, and move in the direction of least resist- 
ance. Let a row of molecules composing a diameter 
be selected, the direction of least resistance would be 
from the center each waj^. For, if surface condensa- 
tion takes place (and experiment shows that it does), 
as the electricity of the molecules near each end of the 
diameter became condensed at the extreme points, its 
reaction being thus neutralized, more would be repelled 
from the center, and this would continue till all the 
electricity of the diameter was condensed at the ends. 

But since the ends are points on the surface, and the 
surface is made up of an infinite number of such points, 
it is evident that the entire charge would be condensed 
on the surface. 

Hence surface condensation takes place under the 



ELECTRIC DISTRIBUTION AND CONDENSATION 57 

influence of attraction from without and repulsion from 
within, in the direction of the radii. 

If the charge be negative, the potential of surround- 
ing bodies being higher than that of the sphere, elec- 
tricity is, in like manner, repelled from the surface 
toward the center ; and the negative charge takes place 
on the surface, as the positive charge did in the first 
instance. Hence the condensation is now in the inte- 
rior, leaving the surface negative. 

Hence surface charge, if positive, takes place under 
the influence of attraction from without and repulsion 
from within ; but, if negative, under the influence of 
repulsion from without. 

In either case the air is the dielectric between the 
electrified sphere and surrounding bodies : and when 
the charge on the sphere is positive, a negative charge 
of corresponding amount is induced on adjacent parts 
of surrounding bodies ; electricity being repelled from 
them by the higher potential of the sphere. But when 
the charge on the sphere is negative, the charge on 
adjacent parts of surrounding bodies is positive ; elec- 
tricity being attracted to them by the lower potential 
of the sphere. 

Now since surrounding bodies, as a whole, are at 
zero, and this positive charge, in their adjacent parts, 
results from the negative attraction of the sphere, it 
is evident that the interior potential of the sphere, as a 
whole, cannot rise above zero; the negative potential 
of its surface being exactly equal to the positive of 
adjacent parts of surrounding bodies, just as their 
negative potential was equal to the spline's positive 
surface potential in the first instance. Now, since a 
solid of any conceivable shape could be cut from such a 



58 ELEMENTS OF STATIC ELECTRICITY. 

sphere without altering the electrical conditions named, 
it is evident that, A charge of electricity communicated 
to any solid conductor will be condensed on its surface. 

Surface Transmission. — It is also evident, that 
although a static charge will be thus condensed on the 
surface, electric transmission is not confined to the sur- 
face : since surface condensation is due to induction 
and repulsion, which implies the possibility of trans- 
mission through the substance to reach the surface. 

Hence, although induction operates during transmis- 
sion, it cannot prevent transmission through the sub- 
stance : so that it must not be inferred that the con- 
ducting power is in proportion to the surface, but to the 
mass of the conductor. 

Hence a charge of electricity which could be easily 
transmitted by a solid rod might be sufficient to melt 
a thin tube of the same diameter. 

Hollow Conductors. — The same reasoning which 
applies to an electric charge on a solid sphere will also 
apply to one on a hollow sphere. For if any number of 
the spherical shells composing the interior be removed, 
it does not alter the equipotential of the remaining 
ones, nor of their radii ; neither can it change the induc- 
tion of the outside surroundings. 

And as the form may be altered without changing 
these electric conditions, the same reasoning will apj3ly 
to any hollow conductor. 

Hence, A static electric charge, communicated to a hol- 
loiv conductor, will be condensed on its external surface- 

Proof Plane — But all our conclusions should be 
the result of experiment ; to aid us in which we now 
require the little instrument called the proof plane, 
represented in Fig. 8 ; which consists of a small brass 



ELECTRIC DISTRIBUTION AND CONDENSATION. 



59 



disc, two inches in diameter, to which is attached a 
light ebonite handle, 12 inches long. A light, flat 
spring, which lies close to the disc, its lower end free, 
and its upper end attached to the handle, will be found 
convenient for attaching tin-foil in some experiments. 



a 




Fig. 8— Proof Plane. 

The proof plane is used for examining the electric 
condition of bodies, and for transferring a small charge 
of definite amount. Care should be used to prevent 
the handle from becoming charged, which may happen 
from friction against the clothing or otherwise. 

Experiments with Hollow Conductors. — Let a 
charge of electricity be given to the insulated sphere A, 
Fig. 9, which has an 
opening in the top. In- 
troduce the proof plane 
through this opening, 
taking care to prevent 
contact with the edges ; 
and touch the inside sur- 
face and then the disc 
of the electroscope, with 
it. As the leaves show 
no divergence, it proves 
that the inside is not 

electrified. Fi ^' () ~ II()1,ow Conductor. 

Now touch the outside, and then the disc, and the 
leaves diverge; proving that the charge is on the out- 
side surface. 

Apply the same tests to the insulated cylinder B, and 




60 ELEMENTS OF STATIC ELECTRICITY. 

the same results will follow. And this cylinder may 
be composed either of sheet metal or wire gauze without 
affecting the results. 

Cylinders of the latter kind are often used to protect 
electroscopes from the induction of electrified bodies 
in their vicinity. 

Repeat these experiments, communicating the charge 
to the inside surfaces of the globe and cylinder, and the 
results will be the same ; showing that no charge can 
remain on the inside. 




Fig. 10— Faraday's Bag. 

Bag Experiment. — To test this more thoroughly, 
Faraday constructed a cone-shaped linen bag, shown in 
Fig. 10 ; attached to its mouth a ring insulated on a 
stand, and to its apex two silk cords, by which either 
surface could be turned outward. 

An electric charge was communicated to it, and, on 
testing with the proof plane and electroscope, was found 
to be entirely on the outer surface. The surfaces were 
now reversed, and the charge was found to have been 
reversed also, going to the outside, as before. 

Pail Experiment. — The following experiment by 
Faraday shows the effect of induction on a hollow con- 
ductor : 

Let a tin pail A, Fig. 11, or any similar hollow con- 



ELECTRIC DISTRIBUTION- AND CONDENSATION. 



61 



ductor, be insulated and connected by a wire with an 
electroscope _E r , and let an electrified metal ball B be 
lowered into it by a silk cord. The leaves will diverge 
as the ball enters, and the divergence increase till the 
ball has passed some distance below the edges : after 
which the divergence is not increased by its further 
descent. 




Fig. 11— Pail Experiment. 

If it be lifted out without having touched the pail, 
the leaves will converge, and the ball show no loss of 
charge : but, if allowed to touch while below the edge, 



62 ELEMENTS OF STATIC ELECTRICITY. 

the leaves will remain divergent after its removal, but 
show no increase of divergence by the contact; and the 
ball, after removal, will be found entirely discharged. 
This experiment proves : — 

1. That the induction of the electrified ball has re- 
pelled electricity from the inner to the outer surface of 
the pail if the charge was positive, or attracted elec- 
tricity from the outer to the inner surface if the charge 
was negative ; in either case producing a divergence of 
the leaves. 

2. It proves that induction increases as the ball de- 
scends, shown by the increasing divergence of the leaves, 
till all the lines of force, which can be included within 
the pail, are cut by its surface, after which there is no 
further increase of divergence. 

3. It proves that there is no permanent effect if 
there is no contact ; since the leaves converge when the 
ball is removed. 

4. It proves that the induced charge on the pail is 
exactly equal to the charge on the ball, since no increase 
of divergence occurs from contact, although the entire 
charge has been communicated to the pail, as shown by 
the ball having lost its charge. But this can be strictly 
true only when all the lines of force are cut by the pail ; 
but since some of the nearly vertical lines must escape, 
no matter how deep the ball descends, there must be a 
slight increase of divergence by contact, though it may 
not be perceptible. 

If a charge be given to the pail and the ball be low- 
ered into it by a wire held in the hand, the divergence 
of the leaves, caused by the charge on the pail, will be 
perceptibly reduced as the ball descends. 

This proves that the inner surface of a hollow con- 



ELECTRIC DISTRIBUTION AND CONDENSATION. 63 

ductor can be charged by induction. The charge on 
the pail, if positive, repels electricity from the ball, 
through the wire and hand, to the earth ; or, if nega- 
tive, attracts electricity from the earth; and in either 
case, a certain degree of equilibrium follows, causing a 
corresponding convergence of the leaves. 

Entire convergence cannot be produced, since only 
a small portion of the lines of force from the pail are 
cut by the ball ; while, in the former experiment, nearly 
all those from the ball were cut by the pail. For this 
reason a large ball is best for the second experiment 
and a small one for the first. 

If the ball, in the second experiment, is lowered by a 
silk cord instead of a wire, there is no perceptible effect 
on the leaves, since induction cannot increase nor dimin- 
ish the electricity of the ball when there is no earth 
connection. 

Combination of Patls. — The following experiment 
was made by Faraday with a combination of hollow 
conductors : — 

Let four pails of different sizes be placed on an insu- 
lated support, and arranged one within the other as 
shown in Fig. 12: and let them be insulated from each 
other at bottom by cakes of paraffin, or any other good 
insulator, placed between them. Let silk cords be 
attached to tin- three inner ones, and the outer one be 
connected with an electroscope. 

On lowering the charged ball into the innermost one, 
the leaves diverge as in the first experiment; contact 
between the ball and pail producing no increase of 
divergence, and the ball is then found to be discharged, 
as before: which proves that the interposition of the 
insulated pails, 2 and 3, has not affected the induction. 



64 



ELEMENTS OF STATIC ELECTRICITY. 



Now let pail No. 4 be lifted out by the silk cord, and 
the leaves will converge, and diverge again when it is 
replaced, showing that the charge on the ball was trans- 
ferred to it. 




Fig. 12— Combination of Pails. 

Let a connection be now made by pieces of copper 
wire, let down by silk threads, between each of the 
pails successively, beginning with 4 and 3, till all four 
are in electric connection, and let the effect on the 
leaves be observed as each connection is made. The 
results will be found the same as in the first experiment, 



ELECTRIC DISTRIBUTION AND CONDENSATION. 65 

when but one pail was used: which proves that the 
interposition of interior surfaces has no effect on induc- 
tion ; nor can it prevent the entire charge from going 
to the outside surface when the four pails are in electric 
connection; for if the three inner pails be now removed, 
they will be found to have lost their charge; but there 
will be no change in the divergence of the leaves. 

This experiment is an actual demonstration of what 
has already been stated, that the interior of a solid con- 
ductor, or the shell of a hollow conductor, may be 
regarded as composed of an infinite number of equipo- 
tential shells or surfaces, from which a charge of elec- 
tricity must always pass to the outside surface. 

Faraday's Hollow Cube. — A most remarkable ex- 
periment in this connection was made by Faraday with 
a hollow cube of wood, measuring twelve feet each way, 
covered with tin-foil, insulated and charged by a power- 
ful electric machine. 

He says : " I went into this cube and lived in it, 
using lighted candles, electrometers, and all other tests 
of electrical states. I could not find the least influence 
upon them, or indication of anything particular given 
by them, though all the time the outside of the cube 
was powerfully charged, and la,rge sparks and brushes 
were darting off from every part of its outer surface." 

This experiment verifies the statement made on page 
12 in regard to zero potential; showing that however 
strong the electrification, no indications of electric action 
are perceptible within a space where there is perfect 
equilibrium. So that even if tlie whole earth were as 
powerfully charged, in proportion to its size, as Fara- 
day's cube, we, who live on it, could perceive no electric 
action, if the charge were as uniform as on the cube. 



66 ELEMENTS OF STATIC ELECTRICITY. 

But if it be objected that the case is not parallel, see- 
ing that we live on the surface, it must be remembered 
that we have an atmosphere above us which is a part of 
the earth's matter; so that, although we live on the 
solid surface, we do not live on the outer surface : and 
the surface on which we live is practically equipoten- 
tial over limited areas. 

Faraday, evidently, might have generated electricity 
with insulated instruments, inside the cube, and con- 
densed it on insulated conductors, without either dis- 
turbing the electric conditions by which he was sur- 
rounded, or being prevented by them : just as we do 
without disturbing the earth's electricity, or being pre- 
vented by it. But any connection by a conductor, 
between his instruments and the cube, would have 
caused the charge to disappear; just as a similar con- 
nection with the earth produces the same result. 

Thickness of Electrified Sukface. — The idea 
of surface condensation implies that an electrified sur- 
face must be something more than a mere superficies. 
It must have a certain degree of thickness, the elec- 
tricity penetrating the conductor and surrounding air 
to a certain depth, in proportion to the resistance of the 
air, and the attraction or repulsion of the charge on 
the conductor. Hence the amount of static charge 
which may be condensed on a conductor, per unit of 
surface, depends on the resistance of the air. 

Convection. — It has already been shown that dry 
air is one of the best insulators ; but, since it is a fluid, 
its resistance cannot be so great as that of a solid of the 
same insulating power; for the air molecules, in contact 
with an electrified surface, becoming charged, fly off 
under the influence of repulsion and induction, while 



ELECTRIC DISTRIBUTION AND CONDENSATION. 67 

those farther out rush in to take their place ; creating 
air currents around the conductor, by which its elec- 
tricity is gradually dissipated. The removal of electric- 
ity by the air in this way is called convection. 

Variation of Charge. — Since the insulating power 
of the air varies greatly with its humidity and tempera- 
ture, and since its electric potential is also variable, the 
charge which may be condensed on a conductor will 
vary in like proportion ; dry, cold air being much more 
favorable to the condensation of a hi^li charge than 
damp, warm air ; and air at a high electric potential 
than air at a low potential. 

• Analogous to this is the influence of atmospheric 
pressure on steam ; the temperature varying with the 
pressure under which it is generated. Here pressure 
constitutes resistance, while in the case under consider- 
ation the resistance is due to the causes mentioned. 

Equal electric condensation on every part of the sur- 
face is never practically true ; as the induction of sur- 
roundings varies, and form, as will be shown hereafter, 
lias an important influence. It could only be true of 
an insulated sphere, surrounded by a homogeneous 
medium, and removed from all other influences. 

Influence of Form. — It lias already been stated 
that form exercises an important influence on the 
amount of static charge which may be condensed on a 
conductor; and that a charge on an insulated sphere 
is equally distributed over its surface, when the sur- 
rounding induction is equal: also that the air, by its 
insulation, retains this charge on the surface, and 
by its convection gradually removes it. It is evident 
also that these forces act at equal distances from the 
center. 



68 



ELEMEXTS OF STATIC ELECTRICITY. 




Fig. 13— Spheres in Contact. 



Electrified Spheres.— Let two insulated metal 
spheres, of equal size and similarly charged, be placed 
in contact, as represented in Fig. 13. It is evident that 
either of them, separately, would fulfill the conditions 

just named; but 
when placed in con- 
tact, they must be re- 
garded as one mass, 
having its center at 
the point of contact ; 
the electric distribu- 
tion being the same 
on each. 

Hence the forces 
of induction and re- 
pulsion which before acted to remove electricity from 
the center of the single sphere to the parts most remote 
f rom it — that is, to the surface — now act in the same 
manner, to remove electricity from this new center to 
those parts of the mass most remote, that is, to the 
points A and B, and the surfaces surrounding them. 

There must also be a certain amount of electricity 
distributed over the entire surface of each sphere ; and 
there must be repulsion between the surfaces adjacent 
to the point of contact : so that the charge will be zero 
at this point, and increase each way toward A and B. 

This may be demonstrated by touching the points 
A, B, and with the proof plane, and. after each con- 
tact, bringing it near the disc of the electroscope ; 
taking care to discharge it with the finger before mak- 
ing the next test. 

It will be found that the central point shows scarcely 
a trace of electricity, while the points A and B are 



ELECTRIC DISTRIBUTION AND CONDENSATION. 



69 



strongly electrified. The same test, applied to inter- 
mediate points, shows the charge on them to be in pro- 
portion to their distance from the central point. 

Electeified Cylinder.— Instead of the two spheres, 
we may substitute an insulated metal cylinder, with 
hemispherical ends, provided with pith - ball electro- 
scopes at the ends and center, as represented in Fig. 14. 

A light charge of electricity 
on the cylinder will cause the 
balls at the ends to diverge in 
opposite directions, while the 
central ball will remain un- 
moved, or but slightly affected ; 
showing that the principal part 
of the charge is condensed on 
the ends, and that induction 
and repulsion are operating to 
remove electricity to the points 
farthest from the center, as shown by the position of 
the balls at the ends. 

If a sphere be made to oscillate near one of the balls, 
at right angles to the length of the cylinder, the effect 
of induction will be shown by the ball following the 
movement of the sphere. 

INFLUENCE OF Points. — If a cylinder having cone- 
shaped ends be substituted for the one with hemispher- 
ical ends, dissipatioD of the charge, instead of condensa- 
tion, will occur. For, on the hemispherical ends, the 
charge is retained by the resistance of the air on the 
surface; but the cone-shaped ends terminate in [joints 
which have no surface, hence there can be no resistance. 

But if resistance ls removed, even from a single point, 

it is evident that the entire charge must pass off through 




Fig. 14— Electrified Cylinder. 



70 



ELEMENTS OF STATIC ELECTRICITY, 



that point ; since the removal of electricity from any 
point on a surface creates a difference of potential be- 
tween that and. surrounding points, producing an elec- 
tric movement in the direction of the point of no 
resistance, which must extend to every part of the sur- 
face, and continue till equilibrium is restored. 

Instead of the cylinder with cone-shaped ends, we 
may use one with needles attached to the ends, as repre- 
sented in Fig. 15. A wooden cylinder covered with 
tin-foil can easily be changed in this way. 

It will be impossible to 
charge such a cylinder, even if 
only a single needle be at- 
tached to any part of the sur- 
face. A projecting angle on 
any part of a conductor will 
tend to produce the same re- 
sult. 

Effects somewhat analogous 
to these may be obtained by 
dipping into water a spherical 
body, and also a sharp-pointed 
spike having the same amount of surface. On lifting 
out the spherical body, water will adhere to it, and col- 
lect in a large drop at the lowest part ; being held there 
by adhesion and atmospheric pressure. But if the 
spike be lifted out, point downwards, the water will 
drop off when it reaches the lowest point, there being 
no surface there on which it can be retained by those 
forces. 

Electrified Spheuoid. — If a metal sphere be flat- 
tened at the poles till it assumes the form of an oblate 
spheroid, as shown at A, Fig. 16, the face of a cross- 




-Cylinder with Points 
Attached. 



ELECTRIC DISTRIBUTION AND CONDENSATION. 



71 




S 



C 



J 



section through the poles, as shown at B, will have the 
same form as a cylinder with hemispherical ends. And 
since it has been shown that a charge of electricity on 
such a cylinder is condensed 
on the ends, it is evident that 
a charge on such a spheroid 
will, in like manner, be con- 
densed on its outer edge. 

Electrified Disc. — If a 
flat metal disc, with a thin 
edge, be electrified, the charge 
will go to the outer edge, as in 
the last case. But resistance, 
being in proportion to surface, 
is very small on such an edge, Fig ' 16 " Electrified Spheroid, 
and the charge is rapidly dissipated. Hence such a 
disc, when constructed for the purpose of condensing 

electricity on it, should be pro- 
vided with a round rim, which 
may be called a resistance rim. 

If it be insulated, and there 
be placed on its opposite sides, 
near the edge, two little metal 
stands with pointed stems, on 
which are balanced light metal 
[jointers, having arms of un- 
equal length, as shown in Fig. 
IT. a charge of electricity given to it will cause the 
pointers to arrange themselves in the direction of the 
radii, showing that the electric force is from the center 
outward. 




17— Electrilied Disc. 



CHAPTER VI. 



Accumulators. 



The Charged Pane. — The electric charge which 
may be condensed on the surface of an insulated con- 
ductor is comparatively small, when such a conductor 
r is remote from inductive influence. 

But when another conductor, having a connection 
with the earth, is placed in its immediate vicinity, 
the charge may be greatly increased. 

To prove this, let a sheet of good insulating glass, 

varnished with shellac, be 
coated on opposite sides with 
tin-foil, to within about two 
inches of its edge, and placed 
on an insulating support, as 
shown in Fig. 18. A small 
charge can be given to the 
tin-foil, on the upper surface, 
which will be indicated by sparks passing between it 
and the body from which the charge is given. But the 
limit is soon reached, and no more sparks will pass. 

Now let the lower surface be connected with the earth 
by a strip of tin-foil, and sparks will again pass freely 
between the charging body and the upper surface, till 
a charge greatly in excess of the former is given. 

If the tin-foil strip be suspended with its lower end 
near a conductor, as shown, sparks will pass between 




The Charged Paue. 



ACCUMULATORS. 



73 



it and the conductor, simultaneously with the sparks 
on the upper surface ; indicating that each surface re- 
ceives the same amount of charge. 

But the potential on opposite surfaces will be oppo- 
site. If the upper surface acquires positive potential, 
by an increase of electricity, the same amount will be 
repelled from the lower surface, making it negative. 
But if the upper becomes negative by a decrease, 
electricity, to the same amount, will be attracted to the 
lower surface, making it positive. 

To prove that these charges are equal, let the tin- 
foil strip be removed 
after the plate has been 
charged ; and a wire, 
held by a piece of 
india - rubber tube, to 
insulate it, be bent so 
that its ends come into 
contact with the oppo- 
site surfaces, as shown 
in Fig. 19: a flash and 
report will follow, and both surfaces, after the wire 
has remained in contact for a few moments, will be 
found completely discharged. 

Now, since the removal of the strip produced com- 
plete insulation, perfect equilibrium could occur only 
by the positive of one surface being exactly equal to 
the negative of the other. 

Since induction varies inversely as the square of 
the distance (page 47), it Is evident that, if this factor 
alone is considered, the amount of charge which can be 
given will be in the inverse ratio of the thickness of 
the glass, and hence greater on thin than on thick 




Fig. 19— The Pane Discharged. 



74 ELEMENTS OF STATIC ELECTRICITY. 

glass. But since the resistance of glass is in the 
direct ratio of its thickness, when the specific induct- 
ive capacity is the same, this factor also must be 
considered. 

Hence, in the construction of instruments involving 
these principles, if great sensitiveness and a low poten- 
tial is desired, the glass, or other dielectric, should be 
thin : but if the highest attainable potential is desired, 
there should be sufficient thickness to resist fracture 
or puncture. 

The uncoated margin must also be wide enough to 
make the resistance there equal to that of the thickness ; 
a small fraction of an inch in thickness having a re- 
sistance equal to that of several inches of surface. 

No definite rules can be given, as the resistance of va- 
rious kinds of glass, and other dielectrics, varies greatly, 
as well as the cases in which they may be required. 

As the positive and negative on opposite surfaces are 
equal, it is impossible for a change of potential to occur 
on either surface without a corresponding change on 
the opposite surface. Hence a conductor brought into 
contact with either surface alone will not change its 
potential, unless directly or indirectly connected with 
the opposite surface. Hence the charge on each surface 
is said to be bound by the opposite charge. 

The convection and conduction of the air, so far 
as it can act equally on both surfaces, will in time re- 
store equilibrium. It may also be restored by the oscil- 
lation of a solid bodj T , as a pith ball, suspended between 
conductors connected with both surfaces ; or, by direct 
connection through a conductor, as already explained. 

Instruments constructed for accumulating electricity 
in this way are called accumulators, or condensers. 



A GCUMULA TOES. 75 

The Leydex Jar. — The first discovery of an accu- 
mulator was made by Kleist, a clergyman of Cammin, 
in Pommerania. who stated in a letter to Dr. Lieber- 
kiilm, of Berlin. Nov. 4. 1745, that by pouring a little 
mercury. " spirits," or water, into a phial and con- 
necting it with a nail through the cork, he could 
electrify it through the nail, ignite "spirits of wine"' 
with it, and receive a shock bv touching the nail with 
his finder. 

The same discovery was made in the following year 
in Leyden, by Cuneus, a pupil of Musschenbroek, who 
electrified some water in a flask, which he held in his 
hand, by bringing it into contact with a chain from the 
conductor of an electric machine. On attempting to 
remove the chain with his other hand, he received an 
electric shock which so frightened him that he dropped 
the flask. Musschenbroek, having tried the experiment, 
I lie would not take a second shock for the crown 
of France. 

The discovery created great excitement, and led to 
the construction of improved instruments, to which 
tht 1 name "Leyden jar"' was given. 

The water in this instance constituted the inside 
ting, the hand the outside coating: and, when the 
other hand touched the chain, both surfaces were con- 
nected by a conductor, and a discharge followed, which 
produced the shock. 

Fig. 20 represents the Leyden jar as it is usually 
constructed. The essential elements are two conduct- 
3 separated by a dielectric ; but, for conven- 
ience in charging and discharging, a wooden rap is 
fitted to it. through which passes a brass rod, terminat- 
ing in a hall above, and to its lower end is attached a 



76 



ELEMENTS OF STATIC ELECTRICITY. 



light Spring, or a chain, which comes into contact with 
the inside coating. 

Tin-foil is the usual coating, and is put on with paste, 
covering both surfaces equally to within about three 
inches of the top. Light sheet brass makes a more 
substantial outside coating, and does not require at- 
tachment to the surface. 
It can also be used for 
the inside coating, when 
the mouth is the full 
width of the jar and the 
sides are straight. Sul- 
phuric acid is also some- 
times used for the inside 
coating of jars designed 
for special purposes. 

An instrument called 
a discharger is also repre- 
sented at A, in Fig. 20. 
It consists of a curved 
brass rod, terminating 
in balls, and having an 
insulating handle, of 
ebonite or glass, at- 

Fig. 20-Leyden Jar and Discharger. tached to its Center. It 

is sometimes jointed at the center, and furnished with 
two handles, as represented at B, Fig. 20. Its use is the 
same as that of the bent wire already described. 

The Leyden jar can be made of any insulating 
material capable of being molded into the proper form ; 
but glass seems to be the only substance capable of 
resisting the enormous strain to which the dielectric is 
subjected under a full charge. 




ACCUMULA TORS. 11 

Glass suitable for the purpose must be free from any 
substance which makes it a partial conductor. Hence 
such glass as is commonly used for fruit jars, candy 
jars, and druggist's bottles cannot be used, since it con- 
tains metallic substances. 

Glass of a bright green color, free from bluish tint, 
also the kind known as " hard flint," makes the best 
Ley den jars. 

The Leyclen jar is charged by an electric machine ; 
its inner coating being connected with the machine! 
and its outer coating with the earth, or with the op- 
posite electrode of the machine ; though it is not 
material which coating is connected with the machine, 
except as a matter of convenience. The jar may 
be insulated, and the charge given to the outer 
coating, if the inner coating is connected with the 
earth. 

It is also immaterial whether the charge given is 
positive or negative, as the opposite charge will always 
be induced on the opposite surface ; electricity being 
repelled to the earth when a positive charge is given, 
or attracted from the earth when negative is given. 

The electromotive force (E. M. F.) of a jar is equal 
to the difference of potential between its inner and 
outer coatings. 

CHARGE BY CASCADE. — The method of charge by 
cascade, first proposed by Franklin, is as follows: Let 
a number of jars of equal size, as ^4, jB, C, D, be 
arranged as represented in Fig. '21 ; the outer coating 
of each, commencing with A, being connected with the 
inner coating of the one next to it ; D having its outer 
coating connected with the earth, and .1 having its 
inner coating connected with the machine. And let A, 



78 



ELEMENTS OF STATIC ELECTRICITY. 



B, and C be well insulated on cakes of paraffin or some 
equally good insulator. 

A positive charge given to the inner coating of A 
will induce negative on its outer coating, by repelling 
the same amount of electricity ; and this repelled 
charge must go to the inside of B, since it has no other 
outlet. Hence the inner coating of B will be positively 
charged, and electridiy will, in like manner, be repelled 
from its outer coating to the inner of C. Hence the 
charge of each jar in the series will be similar to that 
of A ; electricity from the outer coating of B being 
repelled to the earth. 




Fig. 21— Jars in Cascade. 

As the energy expended is distributed among four jars, 
it is evident that the charge of each must be much less 
than if the same amount had been expended in charging 
one jar: since the energy accumulated cannot exceed 
the energy expended. But, as the charge is in the 
inverse ratio of the thickness of the glass, the resist- 
ance from this source must increase from A to D, in 
proportion to the number of thicknesses interposed: 
and the charge must vary in the same ratio ; the. neg- 
ative being greatest on the outer coating of A, where 
only one thickness is interposed, and least on the outer 
coating of D, where four thicknesses are interposed ; 



ACCUMULATORS. 79 

the positive on the inner coatings varying in the same 
ratio. The same variation must also occur in the 
resistance of the connectors, and produce a similar 
effect, in a limited degree ; the resistance of a conductor 
being directly as its length. 

If the charge given to the inner coating of A be 
negative, the electric movement is reversed; all the 
inner coatings becoming negative, and the outer pos- 
itive ; electricity being attracted from the earth to the 
outer coating of D. 

The insulations and connections should receive care- 
ful attention, so as to prevent loss by leakage ; which 
will inevitably occur if the insulation is imperfect, or 
if the connectors have points, sharp edges, or projecting 
corners. 

After the charge is given, the jars should be sep- 
arated, placed in connection with the earth, and each 
discharged separately. A single jar, charged to the 
same amount, should then be discharged, and the 
results compared. 

This method will indicate, roughhy, the amount of 
charge of each jar ; but the electrometer, to be de- 
scribed hereafter, will give more accurate results. 

Tin: LiBYDBN Battery. — When a number of jars 
have their inner coatings joined by conductors, and 
also their outer coatings in like manner, the combi- 
nation is called a Leyden battery. 

A convenient form of such a, battery is represented 

by Fig. 22, in which connectors between the inner 

tin-' radiate from a central jar. The outer coatings 

are made of sheet brass, nickel-plated, and screwed to a 

wooden base, their connections being made with copper 

wires attached to the points of the screws underneath. 



80 



ELEMENTS OF STATIC ELECTRICITY. 



This construction for the outer coatings makes them 
durable, gives the jars a firm attachment, and adds 
greatly to the neatness and beauty of the instrument. 

The E. M. F. of a Leyden battery is the same as that 
of a single jar having the same amount of coated sur- 
face. There can be no increase of intensity from any 
special arrangement of the jars, as such a battery is 
merely an accumulator, and not a generator of electric- 




Fig. 22— Leyden Battery. 

ity. But when great E. M. F. is required it is generally 
more convenient to use a battery than a single jar of 
equal energy. And, in case of fracture from an over- 
charge, a small jar can be replaced at less expense than 
a larger one. 

In charging or discharging a battery, it is immaterial 
which jar°is selected : for all the inner coatings being 
connected together, as well as all the outer coatings, 
each is practically the same as a single coating of equal 



ACCUMULATORS. 



81 



size ; and connection with any part of either coating 



affects the whole of that coating. 



Discharge Through a Book. — The discharge 
from a Lej^den jar or battery, passed through a card or 
a thin book, leaves a puncture, with a burr projecting 
from each surface. 




Pig. 23— Discharge Through a Book. 



To perform this experiment successfully, let one 
knob of the discharger be placed in contact with the 
outer coating, and the other in contact with the book; 
and let the book, held by its edge, with the knob 
against it, be brought quickly into contact with the 
knob of the jar, and the discharge will take place as 
shown in Fig. 23. 



82 ELEMENTS OF STATIC ELECTRICITY. 

Iii this way a book of one hundred or more pages may 
be perforated. 

If the book is first placed in contact with the knob 
of the jar, part of the charge will escape from the edges 
and corners of the leaves, and the experiment is liable 
to fail. 

The burr projecting from each surface, after the dis- 
charge through a book or card, has been relied on as a 
proof of the dual nature of electricity, and ascribed to 
the rush of positive and negatiye in opposite directions. 
It is also attributed to the expansive force of heat, or of 
gas, generated by the discharge. 

The first theory cannot be accepted, unless we have 
stronger proof of the dual nature of electricity than is 
afforded by this experiment. And the second also fails ; 
since in the case of a discharge through a book, the 
leaves may be held so loosely as to allow a free outlet 
for expansion from heat or gas, and yet the burr turns 
in opposite directions from a point near the center of 
the book, and becomes more prominent when the leaves 
are thus held than when they are compressed ; whereas, 
if the burr were due to the expansive force of confined 
heat or gas, the reverse would be true. 

Since these theories are unsatisfactory, let us en- 
deavor to explain this phenomenon in accordance with 
the principles which we have been considering. 

Let a jar be charged on its inner coating, and 
discharged through a book, as represented in Fig. 23. 
Suppose the charge to be positive, electric movement 
being from higher to lower potential, it would be from 
the knob of the jar to the nearest knob of the dis- 
charger. The entire charge of the inner coating, 
passing out through the knob, would induce a high 



ACCUMULATORS. 83 

negative potential on that point, on the nearest surface 
of the book, in a line between the knobs ; repelling the 
electricity of the book along that line to the opposite 
surface, which would thus become highly positive. 

The paper being a very imperfect conductor, the 
charges thus induced do not spread rapidly, but remain 
concentrated for a moment on small circular spaces 
around each of these points ; the greatest intensity 
being at the centers. Hence there is a powerful at- 
traction between the knob of the jar and this negative 
point on the surface of the book ; and also between the 
knob of the discharger and the positive point on the 
other surface ; under the influence of which the paper 
on each surface gives way and bursts outward toward 
the knobs; that surface next the knob of the jar being 
attracted, and that next the knob of the discharger 
repelled. 

As each outward leaf bursts, the next, becoming 
then the outer one, bursts also, till the perforation is 
complete from the center each way. All of which 
occurs instantaneously. 

Meantime the electricity from the knob of the jar 
follows up this inductive effect on the electricity of the 
book; but meeting great resistance from the imper- 
fectly conducting paper, and the air between the leaves, 
it is concentrated on each leaf successively; so that the 
inductive force is constantly in advance of the charge, 
the leaves and layers of air between them constituting 
the dielectric. 

It will be noticed, then, that tins is not a case of 
energy going through a passive medium, but oienergy 
acting on the energy of that medium, causing it to become 
active and perform zvork. 



84 ELEMENTS OF STATIC ELECTRICITY. 

It should also be noticed that when the leaves are 
held loosely, the thickness of the air dielectric is in- 
creased ; each laj'er of air having a charged surface of 
partly conducting paper on each side of it, is in the 
position of the coated pane, a powerful attraction 
between the surfaces acting across it. And when the 
paper bursts there is more room for the formation of a 
burr, and less resistance to the tearing of the paper, 
which accounts for the increased prominence of the 
burr. 

If the charge of the jar is negative, the same results 
occur in reverse order. 

The Residual Charge. — When a Leyden jar is 
discharged, there still remains a slight difference of 
potential between the coatings, which is known as the 
residual charge. Hence, a small discharge can be 
obtained a moment after the first ; and this also leaves 
a residual, bearing about the same proportion to the 
second discharge as the second to the first, when the 
same length of time elapses between them. A number 
of successive discharges may thus be obtained, which 
constantly decrease in amount till no further discharge 
is perceptible. But, even then, it is not probable that 
perfect equilibrium is restored. 

To understand this, we must remember that even 
the best dielectric is a partial conductor: and that 
while electric movement is instantaneous in a good 
conductor, it is very slow in a non-conductor. In the 
Leyden jar we have a combination of both — two con- 
ductors separated by a non-conductor. And, when the 
charge is given, every part of each coating instantly 
becomes electrified, one coating positively and the other 
negatively, on the surfaces next the glass. 



ACCUMULATORS. 



85 





The electricity, on the positively electrified coating, 
slowly penetrates into the glass, acting inductively on 
its electricity, which it repels from the opposite sur- 
face ; and producing, probably, a temporary strain or 
distortion of its structure. 

When the first discharge takes place, there 
is a relief from this strain ; and, as the 
electrified glass slowly returns to its former 
state, the electricity which had penetrated it 
returns to the conducting surface. 

This view receives confirmation from the 
fact that delay increases the residual charge, 
giving time for the electricity to come out of 
the glass and accumulate : while it has the 
opposite effect on the primary charge, reduc- 
ing it by giving time for dissipation. 

Tapping the jar lightly hastens the in- 
crease of the residual charge, the vibratory 
motion thus given to the glass tending, prob- 
ably, to relieve the electric strain. 

Jar with Movable Coatings.— If aJrithM^bfe 
Leyden jar be constructed with any rigid Coatin » s - 
metal, as sheet brass, for both coatings, as suggested on 
page 76, and the conducting rod be attached to the 
inner coating, the coatings may be removed and re- 
placed at pleasure, as represented in Fig. 24 : and we 
have the means of investigating certain phenomena in 
regard to the electrification of the differenl parts. 

Lei a charge be given to such a jar, and the coatings 
removed carefully, so that they .-hall not be connected 
by a conductor during removal: they may now he 
brought into contact without producing any electric 
effect : and the jar also may be handled with a like 




86 ELEMENTS OF STATIC ELECTRICITY. 

result : but, on replacing the coatings, a full discharge can 
be obtained, the same as if they had not been removed. 

But if, while the coatings are removed, the jar be 
examined by touching both surfaces with the finger and 
thumb, or a small discharger, made with a bent wire, 
at any point below a line marking the position of the 
upper edges of the coatings, a discharge can be obtained 
from that point. In this way a number of small dis- 
charges can be had from various points, but no general 
discharge. 

This proves that the charge remains on the glass, while 
the coatings are removed ; but that the resistance of the 
glass prevents a general discharge. But it cannot be 
accepted as proof that the charge is confined to the 
glass, when the coatings are in contact with it ; unless 
it can be shown that the charge remains on the glass 
after the removal of both coatings at precisely the same 
instant ; which could not be done with the care neces- 
sary for so delicate an experiment. But when the 
coatings are removed separately, the charge must be 
transferred to the glass during the removal of each : 
since it is impossible to produce any change of poten- 
tial on either surface, unless a corresponding change 
is produced, at the same instant, on the opposite surface; 
each being bound by the opposite. 

Various Effects of the Discharge. — The dis- 
charge of a Leyden jar of moderate size is sufficient to 
explode gunpowder, and to ignite various substances ; 
as phosphorus, powdered resin, sulphuric ether, and 
alcohol; while that of a large Leyden battery fuses 
wires, magnetizes steel, and destroys animal life. 

With a battery of 550 square feet of coated surface, 
large steel bars have been magnetized, iron wires, T |o 



ACCUMULATORS. 87 

of an inch in diameter, and 25 feet long, melted into 
globules ; and tin wires, T V of an inch in diameter, and 
8 inches long, dissipated in smoke. 

Tyndall accidentally received a charge from a Ley- 
den battery of " fifteen large jars " during a lecture, 
and describes his experience as follows : " For a sensi- 
ble interval life was absolutely blotted out, but there 
was no trace of pain. After a little time consciousness 
returned; I saw confusedly both the audience and the ap- 
paratus. But though the intellectual consciousness of my 
position returned with exceeding rapidity, it was not so 
with the optical consciousness. For my body presented to 
my eyes the appearance of a number of separate pieces. 
The arms, for example, were detached from the trunk 
and suspended in the air. In fact, memory and the 
power of reasoning appeared to be complete long before 
the restoration of the optic nerve to healthy action." 

Gunpowder cannot be exploded by the ordinary 
discharge; the only effect of which is to scatter it. But 
when the discharge is retarded, by introducing into the 
circuit an imperfect conductor, as a wet string, it 
explodes readily. By this method also gun-cotton, 
phosphorus, and other highly inflammable substances 
may be ignited. 

For such experiments the universal discliar</er, rep- 
uted by Fig. 2-"), is convenient. It is constructed 
with a base, in the center of which, mounted on a stem, 
is a small circular tablet of some insulating material, as 
ebonite; and at each end, mounted on insulating stems, 
arc brass sliding rods, each terminating in balls, and 
passing through a socket hinged on the top of its -tern. 
A plaster of paris receptacle, to hold inflammable 
substances, should also be provided. 



88 



ELEMENTS OF STATIC ELECTRICITY. 



The substance to be operated on is placed in the 
receptacle on the tablet, the inner terminals of the 
sliding rods adjusted on opposite sides of it, and the 
outer terminal of one rod connected with the outer 
coating of the jar or battery ; and the circuit completed 
by connecting the outer terminal of the other rod with the 
knob of the jar or battery, by the discharger, as shown. 
The wet string, or other imperfect conductor, when 

used, can be intro- 
duced into any con- 
venient part of the 
circuit, as at S. 

Spontaneous Dis- 
chaege. — A sponta- 
neous discharge is 
iable to occur in 
attempting to charge 
a jar beyond its ca- 

Fig. 25-Universal Discharger. parity : and, if the 

glass is thin at any point, it may be fractured in this 
way; but if the resistance of the insulating margin is 
less than that of the glass, the discharge will take place 
over that surface, without injury to the jar, electricity 
always following the path of least resistance, whether 
longer or shorter. 

Disruptive Discharge. — When a discharge takes 
place through the air or any other dielectric, it is 
termed disruptive; since the electricity must force a 
passage and break down opposing barriers. Such a 
discharge is always accompanied with light, heat, and 
sound; as expressed by the terms spark and snap, flash 
and report — effects due to the resistance encountered, 
and not qualities inherent in electricity. 




ACCUMULATORS. 89 

Silent Dischakge. — But when the discharge takes 
place through a good conductor of sufficient size, it is 
termed silent; since light and sound are absent; the 
resistance encountered being only sufficient to produce 
a slight amount of heat. 

The discharge through a point is also termed silent ; 
since a point, as already shown, offers no resistance ; 
and hence there is little or no sound, even when the 
discharge passes through intervening air. A battery 
discharge, sufficient to destroy life, may be received 
with impunity through the point of a cambric needle, 
held in the hand, without producing any unpleasant 
sensation. 

Lichtenberg's Figures. — If, on a plate of ebonite, 
or of glass varnished with shellac, figures be traced with 
the knob of a positively charged Ley den jar, and sulphur 
dusted over the surface, inclining the plate and tapping it 
to remove the surplus; the sulphur will adhere to the 
lines traced, spreading out in a beautiful fringe, as shown 
in Fig. 26, which is from a photograph of a figure made 
in this way. 

A similar result can be obtained by tracing lines 
with the outside of this jar, or with the knob of a 
negatively charged jar, and dusting the surface with 
red lead. 

( )r a mixture of sulphur and red lead may be used, and 
separate figures traced; the jar being charged positively 
for one figure, and negatively for the other. The sulphur, 
it is claimed, adheres tojhe positive, and the lead to the 
negative lines. Any non-conducting surface may be 
used, also various other powdered substances. 

It should be noticed thai the loss of charge, whether 
positive or negative, from the inner coating, while tracing 



90 ELEMENTS OF STATIC ELECTRICITY. 

the figures with the knob, is balanced by an equal loss 
from the outer coating through the hand in which the jar 
is held. Hence, when the tracing is made with the outer 
coating, the knob must be held in the hand, to produce 
the same effect on the inner coating : the jar being first 
placed on an insulator to prevent a discharge and conse- 
quent shock, by indirect connection through the earth. 




Fig. 26 — Lichtenberg's Figures. 

An inspection of the figure shows, that at the point 
where it begins above, the fringe lines radiate from a com- 
mon center ; but that, as the curve is produced from right 
to left downward and from left to right upward, they 
point diagonally in the direction in which the knob of 
the jar moves. The explanation is as follows : — The 



ACCUMULATORS. 91 

surface being a non-conductor, the electricity has to force 
its way against strong resistance, bursting through at the 
points where resistance is least, and forming the fringe. 
The strongest effect is produced where the knob first 
approaches the surface : as the jar has then a full charge : 
and the first action is a disruptive discharge through 
the air, producing the circular, star-like figure, at that 
point. But as the knob moves along the surface, after 
contact, new lines start out at right angles to the line 
of movement. And as the knob leaves a point where 
such a line has started, it exerts an inductive action on 
the original impulse, which tends to turn this line for- 
ward; the diagonal direction being the resultant of 
these two forces acting at right angles to each other. 
And the forked branches are the result of similar 
inductive action of the main fringe lines on the branch 
lines. 

We have, in this' experiment, a graphic demonstration 
of the effect of an insulating surface in resisting electric 
movement : since the figures show the exact location of 
the electric force ; which, we see, is confined chiefly to 
the tracings, spreading only to the limited extent 
represented by the fringes. 

It also shows that the effects produced in different 
substances, by opposite electric influences, are depend- 
ent on the electric condition of the substances them- 
selves: so that a mixture or a compound may, in this 
way, be separated into its elements. 'Flic sulphur in 
this experiment becoming negative, as claimed, by 
friction, is attracted to the positively charged lines, 
while the red lead, becoming positive, is attracted to 
those negatively charged. This principle has numer- 
ous useful applications in the arts. 



CHAPTER VII. 
Electric Generators. 



The Electrophortts and Fricticxnal Machine. 

The only electric generators noticed thus far are the 
rods of glass, ebonite, and sealing-wax ; rubbed with 
silk, woolen, or fur: but it is evident, that for such 
work as the charging of Leyden jars and batteries, and 
similar experiments, we require generators of far greater 
capacity. But it was thought best to anticipate their 
existence, and defer their introduction till there had 
been a full consideration of the principles on which the 

various kinds de- 
pend: so that they 
might all be in- 
cluded in one com- 
prehensive view ; 
from which the 
merits of each, and 
Fig. 27-Eiectrophoms. the principles of 

its construction could be more fully ascertained. 

The Electrophorus. — This instrument, invented 
by Volta, is one of the simplest forms of a static gen- 
erator ; but it is of great utility in furnishing an 
unfailing, though limited supply of electricity, for 
numerous delicate experiments. 

The following style, designed by the author, and 
represented by Fig. 27, makes a handsome, convenient, 
and very efficient instrument. 




ELECTRIC GENERATORS. 93 

On a wooden base thirteen inches square, constructed 
of layers glued together to prevent warping, is placed 
a thin sheet of brass of the same size ; over which is 
placed a sheet of ebonite of equal size, T V of an inch 
thick ; and both attached to the base by screws near 
the corners. 

On the ebonite is placed a circular plate or cover, 
made of No. 20 sheet brass, twelve inches in diameter, 
perfectly flat, and having a round resistance rim joined 
to the upper surface. In its center is an ebonite handle, 
seven inches high ; and from its rim projects a goose- 
neck, made of No. 8 brass rod, terminating in a half- 
inch brass ball ; near which, on the edge of the base, is 
a brass strip, § of an inch wide, connected with the 
lower plate. 

The base may be made of metal, if preferred, in which 
case the lower plate and strip are unnecessary, the base 
itself taking the place of the plate. 

In this instrument we have two conductors separated 
by a dielectric ; the upper one insulated, and the lower 
connected with the earth. 

The cover being removed, the dielectric is beaten 
briskly with a piece of catskin, or other fur, by which 
its upper surface is electrified ; and the cover is then 
replaced. 

Suppose the charge to be negative ; electricity having 
been removed by the fur, the same amount is attracted 
from the earth to the under surface of the dielectric, 
and to the upper surface of the brass plate in connection 
witli it : which thus become positive by induction. The 
under surface of the cover also becomes positive and its 
upper surface negative. 

Let a connection now be made between the lower 



94 



ELEMENTS OF STATIC ELECTRICITY. 



plate and cover, by touching the strip and knob with 
the finger and thumb, or a small discharger ; the elec- 
tricity accumulated on the lower plate will pass to the 
cover, producing a shock if passed through the hand. 

The cover thus becomes positive ; but its charge is 
neutralized, or bound, by the negative of the dielectric. 
Let it be lifted off by the insulating handle ; its charge 
being no longer bound, a discharge, producing a spark, 

an inch or more in 
length, takes place, 
when the knuckle 
or any conductor 
is presented to the 
knob, as shown in 
Fig. 28. 

The removal of 
the cover with its 
positive charge, 
having left the up- 
per surface of the 
dielectric negative, 
a positive charge 
is again attracted 
to the under sur- 
face and plate, as before ; and the cover, having been 
discharged and replaced, the process may be repeated 
with the same results an indefinite number of times, 
and Leyclen jars charged, or other electric work 
performed. 

Suppose the original charge to be positive, the same 
results occur in reverse order. Electricity having been 
imparted by the fur to the upper surface of the dielec- 
tric, the same amount is repelled from the under 





Fig. 28— Discharge of Electrophorus. 



ELECTRIC GENERATORS. 95 

surface and plate, making them negative. The under 
surface of the cover also becomes negative and its 
upper surface positive. Connection being made as 
before, electricity passes from the cover to the lower 
plate ; leaving the cover negative, and its charge bound 
by the positive on the upper surface of the dielectric. 

The cover being removed, and a conductor presented 
to the knob, a discharge takes place ; electricity now 
passing from the conductor to the cover, instead of from 
the cover to the conductor as before. 

The removal of the cover, with its negative charge, 
having left the upper surface of the dielectric positive, 
electricity is again repelled from the under surface and 
plate by induction : and the cover having been restored 
to zero and replaced, the process may be repeated as 
before. 

We see. then, that when the charge is negative, 
electricity is attracted from the earth to the lower 
plate, then passes to the cover, and then from the cover 
to the presented conductor; but when the charge is 
positive, electricity is repelled to the earth from the 
lower plate ; then an equal amount passes from the 
cover to the lower plate, and the same amount passes 
to the cover from the presented conductor. 

Hence, when the instrument receives a positive charge, 
it gives a negative charge; and when it 8 a neg- 

ative charge, it gives a positive charge. 

It will also be noticed that the initial charge is given 
by friction, but all subsequent charges are obtained by 
induction. 

If the cover be removed, without first making con- 
nection between it and the lower plate, no charge will 
be found on it: since it lias neither gained nor lost 



96 ELEMENTS OF STATIC ELECTRICITY. 

electricity through any external source ; and its own 
electricity, being merely changed to the upper or lower 
surface, by the positive or negative of the dielectric, is 
restored to zero when removed from that influence. 

This connection may be made automatically by plac- 
ing a short brass pin in a hole made through the dielec- 
tric, its upper end even with the upper surface, so that it 
shall touch the cover and also the lower plate. This 
makes the instrument more convenient for obtaining 
charges in rapid succession: but, when used to demon- 
strate the principles involved in its construction, as 
above, the pin should be removed. 

The top of the handle should be grasped, when 
removing the cover, to prevent a partial discharge 
through the hand. 

The electrophorus will retain its charge for months ; 
and, like the Le} r den jar with movable coatings, can be 
taken apart and put together again without perceptible 
loss of charge ; but, when not in use, the charge is 
gradually dissipated, so that only a residual remains. 
Hence it should be charged again before immediate 
use, if great efficiency is desired. This property of 
constancy probably suggests the name, electrophorus, 
electricity-bearer, from (jpeooo to bear, r\lvA.xqov electricity. 

The Frictioxal Machine. — The principle of this 
machine is the same as that of the rod and rubber. It 
was invented b}^ Otto Guericke, and consisted, at first, 
of a globe of sulphur, revolved on an axis by a crank, 
the hand being used as a rubber. Subsequently a globe 
of glass was substituted for the sulphur ; but as insu- 
lation was disregarded in both styles, only feeble results 
were obtained, and the machines fell into disuse. 

Boze, of Wittemberg, revived and improved them, 



ELECTRIC GENERATORS. 



97 



using the glass globe, and a band wheel and belt to 
increase their speed ; and collecting the electricity on 
an iron tube, suspended by silk cords, from which hung 
a chain in contact with the globe. 

Further improvement was made by the use of a 
leather rubber stuffed with hair : and subsequently the 
globe was replaced by a glass cylinder, on one side of 




Fig. 29— Plate Electrical Machine. 

which the rubber was mounted on a glass pillar; and 
on the other side, similarly mounted, was a brass cylin- 
der, called the prime conductor, from which a row of 
points projected toward the glass. An oil silk flap 
enveloped the upper part of the glass cylinder; and a 
chain was used to connect either the rubber or the 
prime conductor with the earth, as desired. 

The plate machine, invented about 1787, was con- 



98 ELEMENTS OF STATIC ELECTRICITY. 

structed on the same principles, a glass plate being 
substituted for the glass cylinder, and has now come 
into general use. Fig. 29 represents one of the prevail- 
ing styles. 

It consists of a disc of plate-glass A, mounted on a 
wooden base with wooden or glass pillars, and revolved 
by a crank with an insulated handle. A pair of rub- 
bers i?, made of soft leather or felt, are pressed against 
the glass on opposite sides by a pair of brass springs (7, 
the pressure being adjusted by a screw. These are 
mounted on a glass pillar, and connected above with a 
brass ball ; and a brass chain, which may be removed, 
connects them with the earth. 

Mounted on a glass pillar is the prime conductor D, 
made of brass, and consisting of a pair of balls, from the 
lower one of which projects a pair of combs, which 
extend on opposite sides of the glass, and whose teeth 
come within a quarter of an inch of it. And, from the 
opposite side of the same ball extends a rod, terminat- 
ing in a small ball. 

A silk cover envelops the lower part of the glass plate, 
and the rubbers, on the surfaces in contact vith the glass, 
are coated with an amalgam, composed of five parts 
zinc, three parts tin, and nine parts mercury, melted 
together, pulverized, and made into a paste with lard. 

The machine should be dry and warm before use, as 
moisture condenses on the surface of the glass when it 
is colder than the atmosphere, and suspends insulation. 
For this reason ebonite pillars have an advantage over 
glass, being less liable to condense moisture. 

Ebonite has also been used for the plate, but is not 
so reliable as glass ; and its liability to warp with heat, 
when in thin plates, makes it very objectionable. 



ELECTRIC GENERATORS. 99 

Its Mode of Actiox. — The plate being revolved in 
the direction of the arrow, electricity is generated by 
the friction of the rubbers; the charged surface of the 
glass passing directly into the silk cover, which prevents 
loss of charge from contact with the air. 

If the charge on the glass is positive, when the 
charged surface comes opposite the combs, electricity 
passes through them from the plate to the prime con- 
ductor, where it accumulates. The glass, being thus 
discharged, passes round again to the rubbers, which, 
having become negative from parting with electricity 
to the glass, have received electricity from the earth 
through the chain. 

Each portion of the plate is thus alternately charged 
and discharged, as it passes first to the rubbers, and 
then to the combs ; the lower half being constantly 
positive, and the upper half at zero, except the resid- 
ual ; electricity passing to the rubbers from the earth, 
and being carried round by the plate to the prime 
conductor. 

If the charge on the plate is negative, the transfer 
takes place in reverse order ; electricity passing from 
the prime conductor to the plate, from the plate to the 
rubbers, and from the rubbers to the earth ; the prime 
conductor becoming negative and the rubbers positive. 

If the prime conductor be placed in connection with 
the earth, by having the chain transferred to it, the 
charge, whether positive or negative, will take place on 
the ball and other parts connected with the rubbers. 

If the prime conductor and rubber be connected by 
the chain, no charge can occur on cither: since elec- 
tricity constantly passes from one to the other through 
the chain, as it is generated. 



100 ELEMENTS OF STATIC ELECTRICITY. 

If the chain be removed entirety, only a veiy limited 
charge can occur, derived from the material of the 
machine itself. 

The limit of the charge is reached when its potential 
energy, whether positive or negative, so far exceeds the 
resistance of the air, that the loss of charge by convec- 
tion, as explained on page 66, shall equal the energy 
generated. When the atmosphere is damp, or its 
electric potential low, this limit is soon reached; but 
when dry, and at a high electric potential, a much 
greater charge can take place. 

Machine Described by Noad. — The largest ma- 
chine of this kind of which we have any record was 
made some years ago for the Panopticon of Science in 
London. According to Noad, it had a plate ten feet in 
diameter, three pairs of rubbers, each three feet in 
length, and a pear-shaped prime conductor, six feet 
in length, and four feet in diameter at its widest 
part- 
It was operated by steam power, and gave sparks 
fifteen to eighteen inches in length; and charged to its 
full capacity, in less than a minute, a Lej T den battery 
of thirty-six jars, having one hundred and eight square 
feet of coated surface. 

Measurement of Energy. — The amount of elec- 
tricity which a well-constructed machine can generate 
is in proportion to the surface area of the plate, which 
may be increased to any practicable limit, the other 
parts being increased in like proportion. It is roughly 
estimated by the number of sparks of a given length 
and energy which can be obtained in a given time, 
when an uninsulated conductor is brought near the 
prime conductor; or by the length of time required to 



ELECTRIC GENERATORS. 



101 



charge a Leyden jar or battery having a given amount 
of coated surface. 

The results are only approximate, especially those 
by the first method, for the following reasons. Length 
of spark is not a true index of energy ; since a short, 
thick spark may have greater energy than a long, thin 
one: and our estimate of the comparative energy of 
each from its appearance, and the accompanying snap, 
is liable to be very inaccurate. The spark accom- 




Fig. 30— Lane's Unit Jar. 

panying the discharge of a Leyden jar or battery is 
generally quite short, though its energy often greatly 
exceeds that of any single spark of much greater length, 
given by the machine in charging it. 

The humidity of the air and its electric potential 
being liable to great variation, produce a corresponding 
variation in the results obtained at different times. 

The charge and discharge of a Leyden jar of a given 
capacity, in a given time, is a more reliable method. 



102 



ELEMENTS OF STATIC ELECTRICITY. 



The jar should be made self-discharging, by bringing 
the knob of a conductor, connected with its outer 
coating, within sparking distance of the knob of the 
jar. 

Lane's unit jar, shown in Fig. 30, is constructed on 
this principle. 

A bent brass rod is connected by a band to the outer 
coating; its upper end terminating in a ball through 
which passes a horizontal sliding rod, terminating in a 
ball at its inner extremity; and having an ebonite 
handle at its outer extremity, by which the ball can 

be adjusted to any required 
distance from the knob of the 
jar. 

To estimate the comparative 
energy of different machines, 
a uniform rotation of the plates 
must be maintained by a given 
number of revolutions per min- 
ute; and the number of dis- 
charges in a given time of the 

Fig. 31— Electric Chime. • , • , ^ •,-! ,i 

& unit jar, connected with the 

prime conductor, will then be approximately correct 
for the energy of each. 

The Electric Chime. — This instrument is used in 
connection with the machine, to illustrate electric 
attraction and repulsion. 

It may be mounted on a separate stand, or hung from 
the projecting rod of the prime conductor. Fig. 31 
represents a common style used in this way. 

It consists of three bells suspended from a brass rod; 
the two outer ones by brass wires or chains, and the 
central one by a silk cord; a brass chain connecting it 




ELECTRIC GENERATORS. 



103 



with the earth. Between the central and outer bells 
are two small brass balls, suspended by silk cords. 

When the machine is put in operation, the outer bells 
receive a charge from the prime conductor; this acts 
inductively on the insulated balls, which are at zero, 
attracts, and, after contact, repels them. Being now 
charged the same as the outer bells, they act inductively 
on the central bell, repelling or attracting electricity 
through its chain, according as their charge is positive 
or negative ; and pro- 
ducing on it a charge 
of the opposite kind, 
they are attracted to it 
and discharged. Be- 
ing now at zero, they 
are attracted to the 
outer bells, as before ; 
and in this way the 
three bells are made 
to ring. 

Image Plates. — 
These are used to 

show the effect of in- Fi S- 32-Iinage Plates. 

duction between two conducting surfaces, as repre- 
sented by Fig. 32. 

From the projecting rod of the prime conductor, a 
brass plate, having a resistance rim, is suspended by a 
wire or chain: and under it. on an insulating stand, is 
placed another similar plate, made a little larger, and 
joined to the insulating support by a sliding rod, by 
which the distance between the plates may be adjusted, 
a chain connecting it with the earth. 

When the machine is put in operation, the upper 




104 ELEMENTS OF STATIC ELECTRICITY. 

plate will have the same charge as the prime conductor. 
If the charge be positive, electricity is repelled by in- 
duction from the lower plate to the earth, through the 
chain ; if negative, it is attracted through the same 
medium ; and, in either case, the plates are oppositely 
charged to the same potential, the air being the dielec- 
tric. 

When the space is properly adjusted, pith balls or 
images, placed on the lower plate, *are alternately at- 
tracted and repelled, dancing up and down between 
the plates in a manner which is often quite amusing. 

If electric connection 
with the earth be sev- 
ered by removing the 
chain, this effect will 
cease : which proves 
that the opposite poten- 
tials of the plates was 
caused by the transfer 
-The El^tric Whirl. of electricity to or from 

the earth, as stated. 
The Electric Whirl. — This little instrument, 
shown in Fig. 33, consists of a set of pointed brass arms 
attached to a common center, which is pivoted on the 
point of a vertical rod connected with the prime con- 
ductor; the arms being bent, so that when passing a 
given point each shall turn in the same direction. 

When the machine is put in operation, the air in 
front of each point becomes electrified, either positively 
or negatively, by the passage of electricity either from 
or to the point; while that back of it is oppositely 
electrified by induction. This causes repulsion from 
the air in front, and attraction toward that at the back, 




ELECTRIC GENERATORS. 



105 



producing rotation of the instrument in the opposite 
direction to that in which the points turn. 

The effect of a stationary point in producing a cur- 
rent of air is shown in Fig. 34; where the flame of a 
candle is represented as blown from a point attached 
to the prime conductor. 

The direction of the air current will be the same 
whether the charge is positive or negative : since, in 
either case, the air embraced within a sphere of which 
the point is the center will have the same potential as 
the prime conductor; while that outside of this sphere 
will assume the opposite potential by induction. Hence 
the air near the 
point becomes 
self-repellent, 
and is also at- 
tracted by the 
air outside ; that 
directly in front 
of the point 
being repelled 
with the greatest force, produces a current in that di- 
rection, while the air on either side is attracted, and, in 
its turn, again repelled. 

Armstrong's Hydro -Electric Machine. — Fig. 
35 represents a machine invented by Sir William Arm- 
strong, about 1840, which generates electricity by the 
discharge of partially condensed steam. 

It consists of a boiler and furnace mounted on glass 
pillars; the boiler being provided with steam and water 
gauges, a safety valve, and a condenser inclosing sev- 
eral small pipes, through which the steam escapes. 

These pipes are surrounded with filaments of cotton, 




4— Air Current from a Point. 



106 



ELEMENTS OF STATIC ELECTRICITY. 



the lower ends of which are immersed in cold water at 
the bottom of the condenser: and the water being thus 
raised by capillary attraction, cools the pipes, producing 
partial condensation of the steam ; thus charging it 
with water in fine drops, by the friction of which 
against the pipes electricity is generated ; the steam 




Fig. 35 — Armstrong's Hydro-Electric Machine. 

being discharged against a row of points connected 
with the prime conductor. 

Each pipe is furnished with a wooden tip : and the 
friction is increased by a tongue of metal, around which 
the steam must pass before entering the tip, as shown 
by the enlarged section at letter A. 

A machine of this kind, constructed for the Roj'al 



ELECTRIC GENERATORS. 107 

Polytechnic Institution in London, had a boiler seventy- 
eight inches long, and forty-two inches in diameter, with 
forty-six steam jets. It gave sparks twenty-two inches 
in length, and charged to its full capacity, in six to eight 
seconds, a Leyden battery, having eighty square feet of 
coated surface. 

Another one, described by Noacl, had one hundred 
and forty steam jets, gave sparks of the same length, 
with three or four times the rapidity; and charged, to 
its full capacity, a Leyden battery having 1,188 square 
feet of coated surface, sixty times in a minute. 

But though capable of such powerful effects, this 
machine is not practical. It is inconvenient to manage, 
requires distilled water, careful cleansing of the boiler 
after use, and great steam pressure. Its operation is 
accompanied with a deafening noise, and the escape of 
a great volume of steam, producing dampness and other 
unpleasant results, when used in a room. Hence its 
chief value is in the demonstration of the important 
fact, that electricity may be generated in this way. 



CHAPTER VIII. 
Electric Generators. 



The Holtz and Topler Machines. 

Influence Machines. — Previous to 1865, frictional 
machines were the principal electro - static generators 
in use. But. that year marked an era in electric prog- 
ress by the invention of two new machines of remark- 
able energy, by the German electricians, Holtz and 
Topler; to which the name influence machines was 
given, from their being constructed with two or more 
glass plates, arranged to generate electricity by their 
mutual inductive influence. 

Both machines are very similar in construction ; the 
principal difference being, tjiat the Holtz requires to be 
incited by an initial charge from an external source, 
while the Topler is self-inciting. 

The Holtz Machine. — This machine, of which 
there are several different styles, is represented by Fig. 
36. On a wooden base are mounted two glass plates ; 
the rear plate B stationary, and supported by three 
ebonite insulators, two below and one above ; while the 
front plate A revolves in the direction of the arrow, 
on a steel shaft, which passes through an opening in 
the center of the plate J?, and is attached to the post 
at M. A is mounted on an ebonite hub, attached to a 
hollow shaft of brass, which revolves on the fixed shaft, 
and carries, at the end next the post, a small pulley, 
from which a belt extends to the driving wheel, which 



ELECTRIC GENERATORS. 



109 



is revolved by a crank with an ebonite handle. The 
relative sizes of the wheel and pulley are such as to 
give the plate four to six revolutions for each revolu- 
tion of the driving wheel, the plates of small ma- 
chines requiring a more rapid revolution than those of 
larger ones. In front of the plate A, £ of an inch from 
the glass, are the combs "Fand H, attached to a brass 
core at the center of the ebonite disc M ; and the 
combs iT and I>, insulated by their attachment to ebon- 




Fig. 3G— The Holtz Electric Machine. 



ite rods projecting from the disc ill", and connected by 
brass rods with the Leyden jars C and D, and with the 
sliding-rods P and R. These sliding-rods have ebonite 
handles, and terminate in brass balls at their inner ex- 
tremities. 

The plates are of sheet glass, about \ of an inch thick ; 
of good insulating quality, and well coated with shellac. 
The stationary plate i?has two circular openings called 



110 ELEMENTS OF STATIC ELECTRICITY. 

windows, directly opposite the combs iT and L ; and, on 
its rear surface, are cemented two paper inductors T 
and X; T extending from H to i, and X from V to K; 
and each armed with a row of points, projecting into 
each window. 

Machines of this kind are often constructed with 
more than two plates ; sometimes with a large number. 
The plates are also sometimes placed in a horizontal 
position. Ebonite plates are also used ; but are objec- 
tionable, for reasons already given. 

The Topler Machine. — The Topler machine has 
the same general construction as the Holtz ; but, on 
the front surface of the revolving plate, are cemented 
a number of small metal discs, called carriers; usually 
made of tin-foil with raised brass centers, which, as the 
plate revolves, are brought into contact with four wire 
brushes ; two attached to the stationary plate, and two 
to the uninsulated combs. In this way the machine is 
made self-inciting, as already mentioned. 

The windows, and the rows of points projecting into 
them, used in the Holtz stationary plate, are omitted 
from the stationaiy plate of the Topler: and the paper 
inductors are made longer, and have small tin-foil in- 
ductors under them, connected, by tin-foil strips, with 
each other and also with the two brushes attached to 
this plate. 

Fig. 37 represents a Topler machine constructed by 
the author, and patented April 10, 1883, and December 
8,1885. The principal points covered by the patents 
are as follows : — 

1. The outside coatings of the Leyden jars C and 
D are of sheet brass, nickel plated ; and are screwed 
firmly to the base ; forming cups into which the jars 



ELECTRIC GENERATORS. 



Ill 



fit closely, and are thus held in a fixed position ; afford- 
ing a firm support to the parts connected with them, 
and preventing liability to accident or injury to the jars 
or plates. 

2. The induced current from these outside coatings 
is conveyed down by the brass screws which attach 




Fig. 37— Atkinson's Topler Electric Machine. 



them, and along copper wires underneath, to the termi- 
nals of the switch S ; through which, when closed, it 
passes from one jar to the other ; but when open, as in 
the cut, it passes by the brass sockets, seen on the edge, 
which are also connected with the terminals, out 
through the conducting cords, and a person, or other 



112 ELEMENTS OF STATIC ELECTRICITY, 

object, connected with their outer extremities. As this 
induced current flows simultaneously with the direct 
current from the inside coatings, the switch and sliding- 
rocls place it completely under control of the operator. 

3. The brush holders, U and F, are attached to the 
plate B, through holes near its edge ; tlius giving a di- 
rect passage to the electricity from the carriers on the 
plate A, where it is generated, through the glass, to the 
tin-foil inductors, represented by the dark shade, and 
the paper inductors T and X, represented by the light 
shade. By passing the electric charge through the glass, 
inside its edge, an insulating margin is interposed be- 
tween the conductors and the edge, thus preventing 
loss from leakage, which is unavoidable when the brush 
holders are attached by clamps or ears on the edge. 

4. The carriers on the plate A are of sheet brass, 
with raised centers, and are nickel plated, making them 
both durable and ornamental. The hard nickel surface 
is not affected by the action of the brushes, or the elec- 
tricity, while tin-foil soon becomes defaced : and the 
carrier, being practically one piece, and its entire sur- 
face cemented to the glass, its raised center cannot be- 
come detached, as may happen when the center is put 
on separately over a tin-foil base. 

5. The combs V and K, also H and i, radiate at an 
angle of 45 degrees to each other, from the central disc 
M, to which they are attached ; so that any possibility 
of error in regard to their position, or of displacement, is 
practically impossible. 

The following improvements may also be noticed: — 
The base is made of two-inch strips, glued together 

lengthways, and heavy cleats screwed on underneath ; 

giving all the advantages of iron as to freedom from 



ELECTRIC GENERATORS. 



113 



warping, with the insulation and elegant finish of the 
wood. The driving wheel is of ebonite and the iron 
casting, on which it is mounted, slides in grooves on an 
iron plate, and is moved by the adjusting screw 0, to 
regulate the tension of the belt. 




Fig. 38— Atkinson's Four-Plate Topler Machine— Front View. 



The ebonite insulators, which support the plate B, have 
soft rubber packing, to ease the pressure on the glass. 

The conducting rods of the Leyden jars pass through 
ebonite caps with cork attached underneath, which 
gives them a fixed vertical position, and affords firm 
support to the sliding-rods and the combs connected 
with them above. 



114 



ELEMENTS OF STATIC ELECTRICITY. 



The Four-Plate Topler Machine. — This machine 
has the same construction in front as the two-plate ma- 
chine as shown by Fig. 38, but a special construction 
for the two rear plates which will be understood by 
reference to Figs. 39 and 40. 

The end view, Fig. 39, shows two pairs of plates, the 
position of the rear pair being reversed, which brings 
the stationary plates into the center, back to back, be- 
tween the revolving plates ; 
so that the inductors are on 
the inner surfaces of the 
stationary plates, and the 
carriers on the outer sur- 
faces of the revolving plates, 
which being mounted on 
the same shaft, with a col- 
lar between them, revolve 
in unison. 

The combs L and K, and 
T^and H, have curved rods 
which pass round the 
plates and support dupli- 
cate combs in the rear as 
shown in the cuts. The 
brushes are also duplicated 
as shown : so that with the exception of the Leyden 
jars and switch, and parts connected with them, this 
is practically a double machine. 

In like manner an eight-plate machine may be made 
by doubling these parts of the four-plate. 

When the large Topler or Holtz machines are 
wanted for constant use, the motive power is usually 
supplied by a steam or gas engine, or a water motor. 




Fig. 39— Atkinson's Four-Plate 
Topler Machine— End View. 



ELECTRIC GENERATORS. 



115 



In which case the driving wheel is not used ; the belt 
passing directly from the small pulley connected with 
the plates, to a pulley attached to the engine or motor. 

Mode of Action of the Toplep. — To compre- 
hend the action of any electric generator, the following 
essential principles in their construction should be kept 
distinctly in view. 

To generate electricity, is to create a difference of 
electric potential ; the efficiency of all generators, 




Fig. 40 — Atkinson's Four-Plate Topler Machine— Rear View. 

whether batteries, dynamos, or glass plate machines, 
depending on the difference of potential which each is 
able to create and maintain within the apparatus itself. 
And the work to be done by such an apparatus is the 
restoration of equilibrium, through an exterior circuit : 
and may consist in producing heat or light, chemical, 
mechanical, or phj^siological action. 

Let us consider how these principles apply to this 
machine. 



116 ELEMENTS OF STATIC ELECTRICITY. 

Fig. 37, page 111, represents the machine with the 
sliding electrodes P and R separated. Suppose the 
switch & to be closed and the machine put in operation. 
It will be seen that as the plate A revolves, the raised 
centers of the six carriers are brought into contact with 
the wire brushes attached to the holders E and F; each 
opposite pair touching opposite brushes, successively, 
at the same instant. The friction generates electricity, 
which diffuses itself over the carriers on A, and the in- 
ductors on B, with which they are, at the instant of 
contact, in electric connection. The potential of car- 
rier and inductor, during contact, will be the same : at 
the next instant the carrier passes on, and is insulated 
from the inductor, and carrier and inductor now act in- 
ductively on each other, and multiply the initial charge 
given by the friction of contact. As it accumulates, it 
spreads over the paper inductors ; these act on the 
opposite surfaces of the glass, till botli surfaces of both 
plates become charged ; the initial charge being still 
continued by the constant friction of the carriers and 
brushes. • 

But, since both sides of the machine are of similar 
construction, and since the mode of action on both 
sides is apparently the same, the question arises, how 
any difference of potential, or electric charge can be 
accounted for. 

And first, it will be noticed, that the position of the 
plates being vertical, their lower halves are nearer to 
the earth, by their semi diameter, than the upper halves, 
and consequently, more under the influence of its in- 
ductive action, by the square of that distance. The 
lower halves are also in close proximity to the Leyclen 
jars, the driving wheel, and the belt, and subject to their 



ELECTRIC GENERATORS. ■ 117 

inductive influence; and the plate B is supported on two 
insulators, while the upper half has but one, and hence 
has the advantage of the better insulation of the air. 

To this lower half of B, and subject to these influ- 
ences, is attached the brush holder F, while B is 
attached to the upper half, and remote from them. 
Hence, the carriers brushed by E, and descending to- 
wards i, must acquire a higher potential than those 
brushed by F, and ascending towards K. 

An accumulation of electricity must also occur at the 
lower ends of the inductors Tand X, from the induct- 
ive influence of the earth ; and as the brush holder F 
is placed at the lower end of X, it furnishes an outlet 
to a portion of this charge, as seen at night by the 
brushes of light from this holder to the outside of the 
jar C\ and other parts in close proximity. 

The lower end of T, on the contrary, is well insu- 
lated ; hence the potential of T 7 , from the heavier charge 
at its upper end, and the better insulation at its lower 
end, must be much higher than that of X, where the 
influences are just the reverse. 

This accumulation, or high positive potential at the 
lower end of 7, produces a high negative potential at 
that point on the plate A, and its carriers, as it revolves ; 
as shown by the brush of light, seen in the dark, from 
the uninsulated comb J 7 , marking the flow of electricity 
to the upper part of the plate, as it passes under that 
comb; the outflow of the current received through the 
comb II This brush of light extends downward, as 
the charge increases, almost to the comb K: and a sim- 
ilar brush extends downward from K, marking the 
outflow of electricity from the interior of the jar (7, as 
explained hereafter: while the points of the combs, B 



118 ELEMENTS OF STATIC ELECTRICITY. 

and H, where the charge is received, show only a glow 
of light. 

These brushes of light always turn in the opposite 
direction to that in which the plate A revolves ; differ- 
ence of potential between the comb and that portion of 
the plate approaching it producing attraction ; while 
equality of potential between the comb and that portion 
of the plate receding from it produces repulsion, (seep. 224.) 

Following any opposite pair of carriers, as IF and Z, we 
find that as Z passes under the wire brush i 7 , W passes 
under E ; and as Z moves on to the insulated comb if, 
IT at the same instant arrives at L ; but W, as already 
shown, has a higher potential than Z, and, at this point, 
a peculiar adjustment takes place. W gives up its 
charge through the comb i, to the inside of the Ley- 
den jar D. This creates a positive charge on the inside 
of D, which induces a negative charge on its outside. 
The electricity thus repelled, passes to the outside of 
(7, making it positive, and inducing negative on its 
inside ; and this repelled electricity flows through the 
comb K to the plate ^4, as already shown. W then 
moves down to the uninsulated comb ZT, while Z moves 
up to V. Each now passes under the wire brush at- 
tached to its respective comb, and the combs being 
attached to the brass core at the center of iff, the carriers 
are put in electric connection with each other, and their 
potential equalized by the flow of the residual charge 
from II to J 7 ", as already described; so* that each arrives 
at the original position of the other at the same poten- 
tial, ready to repeat the same process. 

It should be noticed, that the residual is slightly in- 
creased by induction from T and X, as the carriers move 
from the combs L and if to the combs iTand V. 



ELECTRIC GENERATORS. 119 

The surfaces of the p]ates, on which the carriers and 
inductors are mounted, assume the same potential as 
the carriers and inductors attached to them, while their 
opposite surfaces have the reverse. Opposite parts of 
the same surface are also in opposite electric states : the 
section L M H, for instance, having a potential oppo- 
site to that of V M K; change of potential on these 
surfaces following that of the carriers and inductors, 
already described. 

It will be noticed that the office of the brushes, IE and 
_F, is the reverse of that of if and V. U and F generate 
by friction, while H and V discharge by contact. And, 
while the combs, K and Z, aid in creating a difference 
of potential, the combs, H and J 7 ", aid in restoring equi- 
librium. 

When the difference of potential between the inner 
coatings of the jars becomes sufficient to overcome the 
resistance of the air, a discharge from the inner coating 
of D to that of C takes place between the terminals of 
the sliding electrodes R and P ; and, at the same 
instant, q, discharge from the outer coatings takes place 
through the switch and connections, from C to D, to 
restore equilibrium between them, and thus complete 
the circuit. 

A spark and snap, from the resistance of the air, 
accompanies the discharge between the inner coatings ; 
and the same will occur between the outer coatings if 
the switch is open ; but, if closed, the discharge takes 
place silently. The plates and other parts being, at the 
same instant, relieved of strain, there is a restoration of 
equilibrium in the whole machine. 

The above explanation applies to the machine when 
it is put in operation from a state of absolute rest ; but 



120 ELEMENTS OF STATIC ELECTRICITY. 

when it is in a high state of activity, there frequently 
occurs a reversal of potential after a discharge, as shown 
by the reversal of the brushes of light from the combs. 
To account for this it must be considered, that the 
residual which remains after the primary discharge may, 
from unequal resistance, be greater on one side than on 
the other: and after being relieved from strain by the 
primary discharge, it will operate to give a slight pre- 
ponderance of potential to that side, which is rapidly 
multiplied by induction, as the rotation of the plate 
continues. 

A reversal can also be produced by a temporary 
reversal of rotation, as explained on page 1-10 ; or by 
touching the inductors, or parts connected with them, 
while in action, which would reduce the potential at that 
point. Special conditions may also exist in certain 
machines, which will reverse the ordinary mode of 
action ; as, for instance, a difference of thickness on 
opposite parts of a glass plate ; or in opposite jars. 

It should be noticed that the electric charge is 
instantly diffused over the metal carriers and inductors, 
more slowly over the paper inductors, and still more 
slowly over the shellacked surfaces of the glass plates. 
So that when the machine is put in action, after a con- 
siderable interval of rest, three or four seconds elapse 
before it becomes fully charged, and a crackling sound 
is heard from the electricity forcing itself over the 
resisting surfaces of the paper and glass. 

The condition of the air, as to its insulation, influ- 
ences the whole operation of this machine. An air 
space insulates the plates, and also the jars, with their 
rods and balls, from each other; and as a damp atmos- 
phere lessens this insulation, it will decrease the energy 



ELECTRIC GENERATORS. 121 

of the machine in like proportion. A film of moisture, 
settling on the plates, will often so reduce the insula- 
tion, that the slight initial charge bj the action of the 
brushes is conducted over the damp surface as fast as 
it is generated ; so that no difference of potential, and 
consequently no permanent charge, can occur. And as 
the machine is much more sensitive to such influences 
than the operator, the latter is often puzzled to know 
why it will not generate. The simple and effectual 
remedy, in all such cases, is to dry it. This may be 
done by a fire, a kerosene lamp, a hot iron, or by 
the sun's heat, though artificial heat is generally more 
effectual. 

Warm days, before or after rain, when the atmos- 
phere is loaded with moisture, are the most unfavorable. 
At such times the plates should not only be dried, but 
warmed, as moisture will continue to be deposited so 
long as they are colder than the air. 

The electric conditions in upper rooms, other things 
being equal, are more favorable to the operation of the 
machine than in those on the ground floor. 

Multiplication of the Chaege. — The multipli- 
cation of the initial charge proceeds with great rapidity. 
During the first revolution of the plate A, each tin-foil 
inductor receives six direct charges from the contact of 
its connecting brush with each of the six carriers: and 
also six inductive charges of equal amount, as each 
charged carrier passes it. So that at the end of the 
first revolution, it has accumulated twelve charges ; and, 
during that revolution, it has reacted inductively on each 
passing carrier with this constantly increasing energy, 
increasing the energy of the carrier in like proportion. 

At the beginning of the second revolution, it has 



122 ELEMENTS OF STATIC ELECTRICITY. 

twelve times the inductive energy which it had at the 
beginning of the first ; and this energy continues to 
increase, and react op the carriers, at the same rate as 
before. And as the plate makes about five revolutions 
per second, the rate of increase on any tin-foil inductor 
is about sixty increments per second. 

But as the charge spreads from the tin-foil inductors 
over the paper inductors and adjacent parts of the sta- 
tionary plate; and from the carriers over adjacent parts 
of the revolving plate, each point on each plate, within 
the charged areas, becomes a center of direct and induct- 
ive action in the same manner as the metal inductors and 
carriers. So that even an infinitesimal charge is increased 
in a few seconds to the full capacity of the machine. 

HOLTZ AND ToPLER* MACHINES COMPARED. — Since 

the chief difference between the Holtz and Topler con- 
sists in the latter being self-inciting, the mode of action 
is essentially the same in each. 

The" Holtz may receive its initial charge from a fric- 
tional machine, an electro phorus, or any similar, exter- 
nal source : but the usual method of charging is by 
means of a piece of ebonite, electrified by the fur of a 
cat-skin. 

The electrified ebonite is held in contact with one of 
the paper inductors on the stationary plate, which is 
thus charged ; a portion of the charge being commu- 
nicated to the revolving plate through the points which 
project into the windows ; and this plate is made to 
rotate rapidly, so that the charge is soon multiplied to 
the full capacity of the machine, if the atmospheric con- 
ditions are favorable; and the ebonite is then removed. 

It will thus be seen that the initial charge in both 
machines is produced by friction and multiplied by 



ELECTRIC GENERATORS. 123 

induction. In the Holtz it is derived from an external 
source, begins on the stationary plate, and is then com- 
municated to the revolving plate. In the Topler it is 
produced by the machine itself, begins on the revolving 
plate and is then communicated to the stationary plate. 
In the Holtz it occurs on one side only. In the Topler it 
is simultaneous on both sides. In the Holtz it ceases when 
the plates are charged. In the Topler it is continuous. 

The absence of the brushes, carriers, and metal in- 
ductors from the Holtz increases the internal resist- 
ance, making it more difficult to charge, but giving 
better insulation, and consequently greater energy than 
a Topler of the same size. 

But the action of a Holtz is much more liable to 
interruption from dampness, and a low electric poten- 
tial in the atmosphere : since it receives only a small 
initial charge, which is soon discontinued ; while that 
of the Topler is constant, from the continuous action 
of the carriers and brushes. So that a well constructed 
Topler, with ordinary care, is reliable in any state of 
the atmosphere, while a Holtz is very unreliable. 

Comparison by Dr. Holtz. — In reply to an 
inquiry as to whether the Topler machine was an 
original, independent invention, or only a modification 
of the Holtz, the author received a letter from Dr. 
Holtz, written from Greifswald, Germany, March 20, 
1883; in which he says, that his machine, as first de- 
scribed in Poggendorffs Annalen, in 1865 (volume 
125, page 469, and volume 126, page 157), had "two 
discs rotating in opposite directions, without stationary 
discs " ; and that " The Topler machine, invented at the 
same time, was a combination of two pairs of discs "; 
two movable and two stationary. 



124 ELEMENTS OF STATIC ELECTRICITY. 

He then says : — 

" Topler has recently rejected his system and adopted mine, 
because it is simpler, and, at the same time, more effective. The 
application of the pointed combs and the non-covered movable 
discs is also my invention, since the Topler machine had only the 
tin-foil coverings and sliding springs. (Schleifende Federn.) 

" I had been accustomed to the same, indeed, already : although 
not with independent acting, influence machines, but rejected 
them on account of the smaller spark-length. 

" Topler has also lately adopted my principle of the pointed 
combs, and the non-covered discs : but so far modified, that besides 
the pointed combs and non-covered discs, he yet allows to act. at 
the same time, small pieces of tin-foil (or pieces of metal), and the 
sliding springs. This has the advantage that the machine excites 
itself, and is less sensitive to moisture: but also the great disad- 
vantage, that the sparks become shorter, and a constant reversal 
of current follows. Besides, a certain mechanic. Voss. also claims 
this machine, so modified, as his merit; but unquestionably Topler 
was the first who showed that influence machines, with metallic 
covering and sliding springs, excite themselves. 

"The entire form of the machine, its symmetrical construction, 
the one-sided support of the axis, the application of a sheath 
running upon a pin fastened on one side, the application of the 
so-called rotary diametrical (double) pointed combs, the applica- 
tion of the so-called condensers (small Leyden jars) for increase of 
spark-length, is all mine, as published in the year I860, by Professor 
Poggendorif (Poggendorffs Amuden, vol. 136, page 171). 

" Yours truly. Dr. W. IIOLTZ." 

The " sliding springs " mentioned above, doubtless 
refers to a style of construction in which the springs 
glide continuously over the surface of the glass ; essen- 
tially different, and differing in its effect, from that of 
the brushes, which touch only the raised centers of the 
carriers, and are wholly insulated from the glass; 
giving alternate contact and insulation, making induc- 
tion much more effective. The latter construction is 
attributed to Voss. 



CHAPTER IX. 
Experiments with the Topler Machine. 

In experiments with the friction al machine, such as 
the charging of Leyden jars, and .the ringing of bells, 
as already described, induction is produced by connect- 
ing one part of the apparatus with the earth, and 
another part with the prime conductor. But in the 
Holtz and Topler, the charge is accumulated in the 
Leyden jars instead of on a prime conductor; and any 
change of potential in one jar must be compensated by 
a corresponding inductive change in the opposite jar. 
Hence to obtain the full inductive effect, connection 
must be made with the opposite jars. 

For convenience in making this connection, holes are 
drilled in the knobs surmounting the jars, and the 
charge is conveyed by insulated conducting cords, hav- 
ing brass tips which fit these holes. 

Thus, by connecting the inner and outer coatings of 
a Leyden jar or battery with the opposite jars in this 
way, a full charge can be given very rapidly. 

In a similar manner, image plates, bell chimes, and 
other apparatus, mounted on separate stands, can be 
connected and used. 

Electric Chtme for Topler Machine. — Fig. 41 
represents a chime designed by the author, which is 
mounted on the machine itself. It consists of two 
brass arms A and jB, insulated by an ebonite connector 



126 



ELEMENTS OF STATIC ELECTRICITY. 



C ; the tips of the arms being fitted to the holes in the 
knobs of the jars. 

A bell is suspended from each arm by a brass rod ; 
and a brass ball suspended by a silk cord from the 
ebonite connector hangs between them. 

As each bell is at the same potential as the jar with 
which it is connected, the ball is alternately attracted 
and repelled, causing the bells to ring. 

Instruments of this kind have no practical use, except 
to illustrate the principles of the science. 

c Apparent Time of 

the Electric Dis- 
charge an Optical 
Illusion. — The car- 
riers on the revolving 
8 plate of a Topler afford 
special facilities for 
this experiment. They 
are usually six discs, 
arranged in a circle, 
and present the ap- 
pearance of a contin- 

Fig. 41-Chime for Topler Machine. ^^ Wlg]lt rfng ^^ 

the machine is operated in the light ; but when oper- 
ated in the dark, they are seen only when the spark 
renders them visible ; and, instead of the bright ring, 
each appears by itself, apparently motionless, and as 
perfect in form as if really so, just as if the movement 
of the plate were momentarily arrested during the 
passage of the spark. 

This apparent time of the spark may be estimated 
at i second; but if the carriers were really visible 
during that time, the ring-like appearance would be 



i 



r 



EXPERIMENTS WITH THE TOP LEE MACHINE. 127 

unavoidable, as will appear from the following calcu- 
lation. 

Suppose the revolving plate to have an average speed 
of 4| revolutions per second, it is evident that each 
carrier would make a complete revolution in less than 
i second ; consequently if that were the actual duration 
of the spark, each would be continuously visible round 
the entire circle, and hence even a single carrier would 
produce the bright ring. But it is only necessary to 
this result that each should be visible until it takes the 
place of its predecessor — that is during its passage of | 
of the circle, which reduces the time to -is of a second. 
* But if they were visible even half that time, 5V of a 
second, and each were li inches in diameter, and their 
distance, from center to center, 6 inches, we would 
have 6 ellipses, each having a length equal to twice its 
breadth. 

From tlys it is evident that the smallest conceivable 
duration of spark must produce an ellipse : but as each 
presents the appearance of a circle, with no tendency 
to elliptical form, the conclusion is inevitable that the 
apparent duration of the spark is an optical illusion, 
and that its time is so nearly zero, that it cannot be 
estimated. 

We must conclude, then, that at the instant of dis- 
charge the image of the carrier is photographed on the 
retina of the eye, and at the next instant darkness 
supervenes: but the sensation on the retina has a mo- 
mentary duration, during which the carrier appears 
stationary, while in reality it may have passed entirely 
round the circle. 

It is important to notice, in this connection, that the 
appearance and disappearance of the carriers depend 



128 ELEMENTS OF STATIC ELECTRICITY. 

on the rapidity of the discharge ; and when the spark 
is made so short and rapid as to be apparently contin- 
uous, the carriers appear and disappear with each snap, 
like a succession of views in a rapidly moving panora- 
ma, proving that the apparently continuous spark is a, 
succession of sparks so rapid as to give the impression 
of continuity. 

As a flash of lightning is only the same thing on a 
grander scale in nature's own laboratory, we must con- 
clude that the passage of electricity from cloud to cloud, 
a distance often of many miles, is so rapid as to defy 
human calculation. We notice this in chain lightning, 
when the flash, sometimes three to five miles long, is 
seen throughout its entire length at the same instant, 
as if suddenly photographed on the cloud. 

Transmission of Power by Static Electricity. 
— Two machines are necessary for this experiment — 
one called the primary, and the other secondary. The 
secondary should be a very light running machine ; 
hence it is better to make it smaller than the primary, 
and the driving wheel and switch m^j be dispensed with. 

Let the machines be placed near each other, in the 
same relative position, the secondary in front; and 
connected together by conducting cords or wires, joining 
similar pairs of Leyden jars : and let the sliding elec- 
trodes be separated beyond sparking distance. Now 
let the primary machine be put in operation, and the 
movable plate of the secondary will rotate in a direction 
opposite to that of the primary. If the electric energy 
should not be sufficient to overcome the friction and 
inertia, in starting, the plate of the secondary may be 
put in rotation by hand, and its motion will then be 
sustained by the electric action. 



EXPERIMENTS WITH THE TOPLER MACHINE. 129 

The explanation is as follows. When a Topler ma- 
chine is in operation, there is a strong attraction be- 
tween the plates, the result of induction from the 
opposite electric states of the parts in proximity. This 
attraction which constantly increases up to the instant 
of discharge, acts as a resisting force which must be 
overcome by the force used to rotate the plate. Now, 
when the two machines are connected, this electric 
force is transmitted to the secondary, where, having no 
mechanical force to oppose it, as in the primary, 
it causes the rotation of the plate in the opposite 
direction. 

Thus the mechanical force in the primary is trans- 
muted into electric force, passes over to the secondary 
and reproduces mechanical force ; the force applied to 
the primary being expended in the secondary. 

The apparatus thus becomes a scientific bank, with 
its receiving and paying tellers. But nature is a 
shrewd banker, and always exacts full discount; hence 
the mechanical energy, paid in to the primary, is dis- 
counted by friction, leakage, and heat; so that the 
remaining energy may not be sufficient to start the 
plate of the secondary into rotation without an ad- 
ditional payment. 

The sliding electrodes in the secondary machine may 
be adjusted to produce the electric discharge with spark 
and snap, instead of the mechanical rotation of the 
plate ; thus illustrating the transmutation of force, at 
will, from mechanical to electric, and from electric 
either back again to mechanical, or to the heat, light, 
and sound of the electric discharge. 

Source of Electric Supply of the Topler Ma- 
chine. — The earth, the machine itself, and the air are 



130 ELEMENTS OF STATIC ELECTRICITY. 

the only sources from which an electric machine can 
derive electricity. 

With the common friction machine a connection 
with the earth is indispensable, and only a very limited 
charge can be obtained without it ; the transfer being 
either from the earth to the machine, or from the ma- 
chine to the earth, as explained on page 99. Hence it 
is often compared to a pump, drawing electricity from 
the earth through a chain. Kemove the chain and the 
supply ceases. ,x / 

But with the JTopler a similar earth connection 
diminishes the 6harge ; showing a loss instead of an 
increase of charge. Indeed, perfect insulation of the 
generating parts is an essential feature of the machine. 

To demonstrate this more perfectly, let the machine 
be put in operation on an insulated platform, when it 
will be found that there is not the slightest perceptible 
diminution of electric energy. It is evident, then, that 
the earth is not its source of supply. 

A certain amount is, no doubt, obtained from the 
material of the machine itself; but this source would 
soon be exhausted by such experiments as . the charg- 
ing of a large Leyclen battery; whereas such a battery 
may be charged without diminishing the energy of the 
machine. 

The air, then, is the only remaining source, and the 
large amount of ozone generated by this machine is 
conclusive evidence of its electro-chemical action on 
the air, and strong, presumptive evidence that the air, 
thus acted upon, has furnished the electricity whose 
action has changed the oxygen to ozone. 

This would imply that ozone is the result of depriv- 
ing air of a portion of its electricity; whereas if the 



EXPERIMENTS WITH THE TOP LEE MACHINE. 131 

electricity were derived from the earth, we must infer 
that its generation precedes the generation of ozone, 
instead of being coincident with it. But the insulation 
proves that the earth does not supply the electricity; 
so that the weight of evidence is in favor of ozone being 
the direct result of electric generation, rather than a 
result of subsequent electric action. And, if such is 
the case, it is strong proof that the air is the chief 
source of electric supply. 

The generation of ozone by atmospheric electricity 
during thunder storms is a well-known fact; and clouds, 
-floating miles above the earth, must obtain their elec- 
tricity either from their own vapor, or the air, or both. 
Such clouds, at different electric potentials, insulated 
from the earth, acting inductively on each other, and 
finally producing a discharge, fulfill the same conditions 
as exist in the Topler machine ; and the generation of 
ozone is doubtless due to the same cause in both. And 
since the vapor of the cloud corresponds to the material 
of the machine, and it has been shown that the electric 
supply of the machine from its own material must be 
very limited; and since the machine operates most 
effectively in a dry atmosphere, and hence does not 
derive its electricity from vapor ; we may infer that the 
electric action is the same in both cases, and that the 
air is the chief source of electric supply. 

It is evident from the movement of particles of dust 
and other light bodies towards the machine, that the 
air in which these atoms float must have a similar 
movement ; that currents of air are constantly flowing 
to the machine and that this air, after being changed 
to the same electric potential, is repelled, and air at a 
different potential flows in to take its jolace ; a move- 



132 ELEMENTS OF STATIC ELECTRICITY. 

ment similar to that which takes place in the hot and 
cold currents round a heated stove. 

But the initial charge is undoubtedly from the ma- 
terial of the machine itself, and results from the friction 
of the brushes on the carriers ; after which follows the 
increase by induction and the action on the air. 

Electricity Generated by the Friction of 
Metals. — The old division of all substances into elec- 
trics and non-electrics was the exponent of the idea 
then prevalent, that only certain substances, as glass, 
sealing-wax, and other non-conductors, comprised in a 
very brief list, were capable of electric excitation. 
While this view is no longer maintained, yet, since in 
nearlv all experiments illustrating the elements of static 
electricity, glass, sealing-wax, ebonite, silk, wool, fur, 
and other non-conductors, are almost exclusively em- 
ployed as generators, we are apt to lose sight of the 
fact that metals and other conductors are capable of 
generating electricity by their mutual friction. And 
yet this is one of the most important principles of static 
electricity. It is that which liberates our ideas of elec- 
tricity from the narrow bounds to which they were 
once confined, proving that it is not a special property 
of certain substances, but a universal property of mat- 
ter, one form of that energy which pervades and con- 
trols the universe. 

This point has been already illustrated, but the 
Topler machine affords special facilities for illustrating 
it more fully. In it the initial charge is produced hy 
the friction of -metal brushes on metal carriers. True, 
both carriers and brushes are attached to glass, and the 
glass subsequently acts by induction as a generator; 
but the friction is confined to the carriers and brushes 



EXPERIMENTS WITH THE TOPLER MACHINE. 133 

alone; and, so far as the electricity is obtained from 
this source, the glass acts only as an insulator to pre- 
vent the escape of the electricity generated by the 
friction, from which the initial charge is derived. 

It is not even necessary that the metals should be 
different. The machines here described are constructed 
with brass carriers, and brushes of brass wire ; and, 
though the carriers are nickel-plated, so that the friction 
is that of brass brushes on a nickel surface, yet carriers 
left unplated give equally as good results. 

The Spark; Its Direction, Subdivision, and 
Color. — The spark from a Topler machine presents 
phenomena which demand careful investigation. 

As already shown, the apparent time of the discharge 
is an optical illusion, time being practically annihilated; 
so that it is impossible, from observation, to tell in what 
direction the discharge takes place. A brilliant streak 
of white light, extending from one electrode to the other, 
suddenly appears and disappears, leaving us in igno- 
rance as to the direction in which it moves. But the 
following experiment affords better opportunity for 
observation. 

As already shown, the electric connection between the 
inside coatings of the Leyden jars may be interrupted 
by separating the sliding electrodes, and that between 
their outside coatings by opening the switch. Put the 
machine in operation in a darkened room at night, with 
the switch open, and the sliding electrodes separated 
three or four inches. From the electrode P, Fig. 42, 
a brush of violet-colored light, diverging from a small, 
circular space, extends about i of an inch towards the 
opposite electrode, accompanied by a hissing sound. 
The opposite electrode, it, remains comparatively qui- 



134 ELEMENTS OF STATIC ELECTRICITY. 

escent at first, showing only a glow of light ; but, as 
the electricity accumulates, there is a suclcleii outburst 
from it, accompanied by phenomena of the most inter- 
esting and varied character. 

A brush of light, of a faint white, or violet color, 
darts across the intervening space, diverging towards 
the center, and converging as it meets the brush from 
the opposite electrode ; forming an elliptical figure, two 
or three inches in diameter, extending from one elec- 
trode to the other. Through the center of this brush 
shoot out long tongues of red and violet light, curving 
and branching in a variety of fantastic forms. Some- 
times five or six of these appear at once, like fiery 
serpents, hissing, spitting, and darting out their red 
forked tongues. Sometimes the appearance is that of 
a miniature tree, its main trunk branching off at various 
angles and curves. Then, again, the brush disappears, 
and we have a single, straight, violet colored stem, 
about f of an inch long, which divides into a great 
number of bright rays, radiating in straight lines from 
the end of the stem, and forming a globe of white light, 
about three inches in diameter: the whole resembling 
a little bush of remarkably regular appearance, in 
marked contrast with the curved and contorted phe- 
nomena just described. 

Between this, and the short brush on the opposite 
electrode, a dark space intervenes, into which the rays 
pass and intermingle ; the brush from the electrode R 
being largely in excess of the other, and showing far 
greater energy ; but more fitful, coming at first in jets, 
with a spitting sound, while the other is more constant, 
with a steady, hissing sound. 

As explained on page 118, electric movement is from 



EXPERIMENTS WITH THE TOP LEE MACHINE. 



135 



the comb L downwards into the jar D, then along the 
switch, when closed, and its connections to C\ and up- 
ward out of to the plate and carriers ; and, in part, 
alone the electrode P towards It, which connects with 
the inside of D: the glass of the jars, and the air space 
between P and B forming barriers at which electricity 




Fig. 42— Experiments with the Toplei Electric Machine. 

accumulates on the side towards which the movement 
takes place, induction producing a corresponding neg- 
ative on the opposite side. 

D has the higher potential, as already shown, but its 
inside charge is bound by an equal negative on its 
outer coating; while electricity is repelled from the 



136 ELEMENTS OF STATIC ELECTRICITY. 

inside of (7. Hence, when the switch is open, we have 
the difference in the brush discharge already described. 

But as the higher charge of D continues to accumu- 
late on its inside coating, the tension increases on the 
electrode i2, till the electricity finally bursts through 
the resisting air from B to P ; producing the spark and 
snap when the switch is closed, as already explained. 

The effect of opening the switch is to substitute for 
this metal conductor, which has comparatively no 
resistance, a portion of the base, which is of kiln- 
dried wood, and offers high resistance. This retards 
the current, producing the difference of phenomena 
between the bright, instantaneous spark of white light, 
with its sharp report, and the slow moving brushes of 
violet light, with their hissing, spitting sounds; and 
from this slow movement we are able to determine the 
direction of the discharge, as already shown. 

The cause of the subdivision of the spark when the 
switch is open next claims attention. It has been 
shown that the discharge between the inside coatings 
through the electrodes P and li, and the intervening 
air space, is dependent on the counter discharge 
between the outside coatings, through the switch, when 
closed, or, through the kiln-dried wood of the base, 
when the switch is open. This discharge may be seen 
by opening the switch, half an inch or more, so that 
the resistance of the air is less than that of the wood. 
We then have a bright spark below, simultaneous with 
the spark above. But when the switch is opened so 
that the resistance of the air is greater than that of the 
wood, the discharge below takes place silently through 
the wood, and we have above, the subdivided, colored 
discharge already described. 



EXPERIMENTS WITH THE TOPLER MACHINE. 137 

With the switch closed, reducing the resistance below 
to zero, the discharge through the air is instantaneous ; 
and there is seldom any subdivision, except that a long 
spark from a heavy charge sometimes divides into two, 
slightly separated during a part of their course. But, 
with the switch open, the high resistance retards the 
lower discharge, which is compelled to force its way 
slowly through the kiln-dried wood; making the 
change of potential between the outside coatings slow 
and gradual, and producing a similar effect on the 
inside coatings. Now, as the spark is caused by the 
electricity forcing its way through the air, whose elec- 
trified molecules are at the same potential near each 
electrode, and hence self-repellent, while the surround- 
ing air is at a lower potential and attractive, these 
forces, acting in part at right angles to the original 
impulse, during the comparatively slow progress of 
the discharge, produce the brushes of diverging rays 
already described. Various influences, such as currents 
of air, particles of dust, and the induction of electricity 
generated on adjacent parts of the machine, curve and 
contort the spark, producing the peculiar phenomena 
already described in connection with the brushes, and 
also affecting the long bright sparks in a similar manner.. 

We next notice the color of the spark. Light is a 
mode of motion, and its color is influenced by the 
intensity of the motion. A bar of iron, drawn from the 
furnace, ready for rolling or welding, is said to be at a 
white heat; as it cools it changes to a red heat. Here 
the color of the light depends on heat, which is also a 
mode of motion ; and as the intensity of the heat mo- 
tion decreases, the light changes from white to red of 
various shades, till the bar resumes its original color. 



138 ELEMENTS OF STATIC ELECTRICITY. 

The brilliancy of the arc in the electric lamp is due 
to the intensity of the motion, while the softer light of 
the incandescent lamp results from a motion less intense. 
When an electric lamp is being lighted or extinguished, 
the change of color from white to the various shades of 
red is evidently dependent on decrease of motion. 
Must we not 'conclude then that the white light of the 
electric spark, when the switch is closed, is due to 
intensity of motion, and the colored light with the open 
switch, to decrease of intensity, as in the iron bar or 
the carbon of the electric lamp ? Or, if light and heat 
are modes of motion, is not the evidence equally strong 
that electricity is a mode of motion? Or may we not 
go still farther, and say that light, heat, and electricity 
are only different manifestations of that energj' which 
is a universal property of all matter, of which the ex- 
periments here given are an additional proof? For in 
the electric spark, we have light, heat, and electricity 
combined. 

Having stated that D is usually the jar of higher 
potential, it should be explained, that there is fre- 
quently a temporary reversal of potential; and, when 
this occurs, all the phenomena here described are 
reversed also. The cause of this reversal will be ex- 
plained in connection with the following experiment. 

Direct axd Reversed Rotation. — A Topler ma- 
chine can be charged only by revolving the smaller 
plate in a given direction; which, in the machine 
represented, is shown by the arrow. 

The reason is this: In order to store up electricity 
in the Leyden jars, each carrier must pass from an 
insulated brush, where it is charged, directly to a comb 
connecting with a Leyden jar, before it passes to an 



EXPERIMENTS WITH THE TOPLER MACHINE. 139 

uninsulated brush, where it is discharged. Thus the 
carrier TT, charged by the friction of the insulated 
brush E, must pass to the comb Z, connecting with the 
jar D, and give up its principal charge, before passing 
to the uninsulated brush and comb JT, where its resid- 
ual is discharged through the brass rod H V, which 
puts it in electric connection with the carrier Z, of 
opposite potential. Reverse the rotation, and the 
carrier IF, starting from E, would give up its princi- 
pal charge to the uninsulated brush and comb at V, 
before reaching the comb K, connecting .with the 
Ley den jar (7, where only the residual would remain. 
It must also be noticed that the charge is greatly 
increased, both on the carrier and adjacent portion of 
the plate, by passing the inductor I 7 , attached to the 
stationary plate B ; whereas, when the rotation is re- 
versed, the carrier leaves the inductor and passes the 
space between T and X, where the induction is almost 
zero. Thns it is evident that no storage of electricity 
in the Leyden jars, and hence no permanent charge can 
be obtained from a reversed rotation. 

Higher Potential of Jar, D. — It has been shown, 
that from the higher position, and hence better insula- 
tion of the brush J?, and upper half of the revolving 
plate J., as compared with the lower position, and con- 
sequent inferior insulation of the brush F, and lower 
.half of A, the potential of the jar D, receiving its 
charge from the former, must be higher, as a rule, than 
that of (?, which receives its charge from the latter. 

Repeated experiments, made by the author with a 
number of different machines of this kind, fully confirm 
this view. The higher potential is shown by the fre- 
quent partial discharges between the inside and out- 



140 ELEMENTS. OF STATIC ELECTRICITY. 

side coatings of this jar ; and, in case of fracture, which 
sometimes occurs from an overcharge, it is always this 
jar which is broken : and the fracture always occurs on 
the side nearest the opposite jar, showing that the 
charge is attracted to that side, and electricity repelled 
from the outside coating, creating a sufficient difference 
of potential between the two coatings to overcome the 
resistance of the glass and perforate it. 

Reversal of Potential. — It has been already 
stated that there is frequently a temporary reversal of 
potential. Such a reversal can be produced, if desired, 
by joining the electrodes P and i?, and reversing the 
rotation of the plate till the machine is fully discharged; 
then separating P and 11 while the reversed rotation is 
continued, and then resuming the direct rotation, when 
a complete reversal of potential will be found to have 
occurred, which will continue till again reversed by a 
similar experiment, or till the machine has had a period 
of rest. The explanation is as follows: — 

When P and 11 are separated, and the rotation re- 
versed, the same causes which before operated to raise 
the potential of D above that of (7, now operate to raise 
the potential of C above that of i), but in a very lim- 
ited degree. For, as already shown, any carrier, as IF, 
charged by the brush E, would now give up its princi- 
pal charge to the brush and comb at V; but the resid- 
ual, slightly increased by the inductor X, would be 
given up, through the comb iTto the jar O; while the 
opposite carrier Z, would give up its principal charge 
at H, and carry its residual to the comb L, and the jar 
D, after a slight increase by the inductor T. But the 
difference of insulation between the upper and lower 
parts affect these residual discharges in the same man- 



EXPERIMENTS WITH THE TOPLER MACHINE. 141 

ner as the principal discharges, and hence operate to 
make the potential of (7, receiving its charge from 
above, higher than that of Z>, receiving its charge from 
below. This residual is not sufficient of itself to bring 
the machine into action, but it creates a slight differ- 
ence in favor of (7, sufficient to sustain a reversal of 
potential when the direct rotation is resumed. 

The Faradic Current. — The faradic current con- 
sists of a series of electric impulses following each other 
with great rapidity. It is obtained from the battery 
and coil by a spring vibrator, which opens and closes 
the circuit; and from the magneto-electric machine by 
a revolving electro-magnet and commutator. 

Both these instruments have, for many years, been 
extensively used in medical practice ; but the use of a 
static machine for this purpose is quite recent, and the 
switch, on the machine here represented, affords special 
facilities for producing and utilizing this current. In 
Fig. 42 are shown two sockets, on the front edge of 
the base, connecting with the terminals of the switch, 
into which are inserted the tips of conducting cords, to 
the outer extremities of which may be attached metal 
handles, as shown, or other electrodes suitable for the 
use of this current, for medical or scientific purposes. 

As already explained, when the machine is in oper- 
ation there is a constant movement of electricity 
through the switch and its connections, from I) to (7, 
while the charge is accumulating; and the counter dis- 
charge through them, from C to 7), is simultaneous with 
the discharge above, from R to P. When the switch is 
open and the cords attached, as shown, this discharge 
must either force its way through the kiln-dried wood, 
or pass out through the cords and any object connected 



142 ELEMENTS OF STATIC ELECTRICITY. 

with their outer terminals, according to the degree of 
resistance offered by each path respectively. If a per- 
son, or a number of persons with hands joined, grasp 
the handles, the resistance will be less than through the 
wood, and they will feel the effects of the discharge. 
This discharge is regulated by the distance to which R 
and P are separated. With a separation of T V of an 
inch, on a large machine, the discharge is so rapid that the 
distinction between the impulses can scarcely be per- 
ceived; producing a faradic current smoother than can 
be obtained from the best batteries, while a separation 
of 2 an inch produces effects which the strongest nerves 
cannot endure. 

This current, in its milder form, cannot be distin- 
guished from that obtained from the battery, or mag- 
neto-electric machine : but, in its more powerful effects, 
it is more impulsive; coming in jets, with cumulative 
force, like the rapid blows of a planishing hammer. In 
the battery current, the stronger effects show increased 
intensity, and a greater tendency to muscular contrac- 
tion ; while increase of strength in this current is 
due to the slower impulses giving more time for the 
accumulation of electric energy. 

The Electric Bath and Electric Wind. — Charg- 
ing a person on an insulated stool is one of the most 
common experiments in static electricity, but it has 
only recently come into use in medical practice; and, 
instead of the stool, an insulated platform, on which 
one or more persons can be comfortabty seated, has 
been substituted; the treatment being known as the 
"Electric Bath." 

When the patient is seated, as above, the electrodes 
P and i£, drawn out beyond sparking distance, and the 



EXPERIMENTS WITH THE TOPLER MACHINE. 143 

switch closed, a connection is made between the pa- 
tient and the machine by a conducting cord; one end 
being attached to the ball surmounting one of the 
Leyden jars, and the other end to the chair. A similar 
connection is made between the opposite jar and the 
floor near the platform, to create a certain degree of 
induction, and so facilitate the process of charging, 
which is now done by putting the machine in oper- 
ation. Very little sensation is experienced from this 
charge, but its effect in certain nervous diseases, which 
cannot be treated with the battery, such as St. Vitus 
dance, is said by medical men to be very soothing. In 
other cases, sparks are drawn from the patient with the 
hand or a suitable electrode, as a ball, roller, or sponge, 
attached to the cord from the opposite jar, and held by 
an insulating handle. 

The electric wind is given by a point electrode, 
attached as above, either with or without the insulated 
platform. A gentle current of electrified air from the 
point fans the patient, producing a delightfully sooth- 
ing sensation. 

Electric treatment of this kind can be given only by 
static electricity, and its value must be determined by 
the medical profession, among whom it is coming into 
favor; being used and recommended by physicians of 
eminence. 

Gas Lighting. — Lighting the gas in churches and 
public halls by electricity is commonly done by a bat- 
tery and coil, but the Topler machine can also be used 
for this purpose. With either method there must be 
wires connecting the generator with the chandeliers, 
wires connecting the chandeliers together, and also the 
separate burners; all arranged in one circuit and prop- 



144 ELEMENTS OF STATIC ELECTRICITY. 

erly insulated. At each burner there is a break in the 
circuit, so arranged that a short spark will pass through 
the gas; the ends of the wire being attached to an 
insulator fitted to the burner. 

With the battery there is a ground wire, and con- 
nection with the gas pipe to complete the circuit; but, 
with the machine, the circuit is made by two separate 
wires, connecting the chandeliers with the balls sur- 
mounting the Leyden jars. On account of the greater 
intensity of static electricity, these wires must be thor- 
oughly insulated with thick rubber tubing, wherever 
they are liable to come in contact with the walls or gas 
fixtures. With these arrangements properly made, it 
is only necessary to close the switch, separate P and R 
to the full extent, turn on the gas, and put the machine 
in operation. The resistance of the air between P and 
i?, being greater than the resistance of the wires and 
the short breaks between their terminals at the burners, 
the sparks take place at the burners, and the gas is lit. 

As to the expense, convenience, and efficiency of this 
system, as compared with the battery system, only gen- 
eral statements can as yet be made. The first cost 
would probably be about the same; after which there 
would be no further expense with the machine, which, 
with proper care, should remain in good working order, 
for this purpose, for an indefinite term of years; while 
the battery requires frequent renewal of the fluid, and 
occasional renewal of the zinc, besides cleansing and 
amalgamating. 

As to efficiency, the greater intensity of the spark 
from the machine will be evident, when we consider 
that a machine of very moderate size will easily pro- 
duce sparks three to five inches in length, while a very 



EXPERIMENTS WITH THE TOPLER MACHINE. 145 

large battery and coil would be required to produce the 
same result. But this should be taken merely as an 
indication of comparative intensity ; as, practically, only 
very short sparks are required : so that a battery and 
coil of medium size is generally sufficient. 

A damp atmosphere does not affect the battery, 
while it lessens the energy of the machine; and, in 
unskillful hands, may interfere with its practical 
efficiency. But, with either system, the person in 
charge should have a thorough knowledge of its care 
and management: in which case the machine can 
always be kept in practical working order. 



CHAPTER X. 
Electric Transmission in Vacua. 

Electric Transmission in Low Vacua. — Let a 

glass tube, about thirty inches long, be provided 
with brass caps at each end, fitting air tight; from 
each of which a pointed brass rod projects inwards. 
And let a stop-cock be attached to one of the caps, by 
which the tube can be connected with an air pump, as 
shown in Fig. 43. 

Let the tube be insulated, and the caps connected by 
conducting cords with the balls surmounting the Ley- 
den jars of the Topler machine; the sliding electrodes 
being separated to their full extent. When filled with 
air, at the ordinary atmospheric density, it will be found 
impossible to pass an electric charge through a tube of 
this length: but let it be connected with an air pump, 
and the air well exhausted, and a charge will easily pass 
through. This proves that air at the ordinary density 
lias a much higher electric resistance than rarefied air. 

But if a high degree of vacuum is produced, it will be 
found much more difficult to pass the charge through : 
which indicates that a medium, consisting of some 
material substance, is essential to electric existence 
and movement; and that if it were possible to produce 
an absolute vacuum, electricity could not pass through. 

If the above experiment be performed in a dark room, 
flashes of red and violet colored light will be seen to 



ELECTRIC TRANSMISSION IN VACUA. 147 

accompany the discharge, strongly resembling the cor- 
uscations of the aurora polaris. Hence tubes, used for 
this purpose, have been called aurora tubes. 

Geissler Tubes — Improved tubes of this 
kind, called from their inventor Geissler tubes, 
are constructed with fine platinum wire sealed 
into their extremities; the points projecting 
inwards, and loops formed outside for the 
attachment of conducting cords or wires. The 
glass is bent into a variety of graceful curves 
and folds: small tubes, bent in this manner, 
being inclosed, for protection, in large straight 
ones; and thus long, frail tubes are reduced to 
compact, convenient forms, in which they can 
be safely handled, as shown in Fig. 44. 

The air is exhausted from them by a mercury 
pump, after which they are hermetically sealed. 
The expansion of the fine platinum wires being 
very slight and nearly the same as that of the 
glass, is not sufficient to cause fracture, hence 
the vacuum produced in well-constructed tubes 
remains permanent for years. 

Beautiful fluorescent effects are obtained by 
constructing such tubes of uranium glass. Sim- 
ilar effects are also obtained by introducing into 
them. various solids and gases; as sulphate of 
quinine, fluoride of boron, fluoride of silicon, (§§ 
iodine, hydrogen, and nitrogen ; which give 
certain characteristic colors, when subjected to 
electric action. £ j g. 43— 

Vacuum 

The effect of the discharge is greatly increased Tube - 
if a break be made in the connection between one end 
of the tube and the machine, so as to introduce a short 



148 ELEMENTS OF STATIC ELECTRICITY. 




Fig. 44 — Geissler Tubes. 



ELECTRIC TRANSMISSION IN VACUA. 149 

spark into the circuit. The electricity then accumulates 
and a rapid succession of brilliant discharges is the result. 

The same effect can be produced by opening the 
switch, connecting the tube with its terminals, and 
slightly separating the sliding electrodes P and P. 

When the spark between P and R is apparently 
continuous, the pulsations in the induced discharge 
through the tube are distinctly visible in the alterna- 
tions of light and shade ; proving that the discharge 
consists of a series of distinct impulses. 



Fig. 45— Rotary Movement in High Vacua. 

Electric Transmission in High Vacua. — The re- 
sidual air in the ordinary Geissler tube is about -nfiftfflW 
of an atmosphere, but Crookes has produced tubes in 
which the residual is less than T oo<W<7o of an atmos- 
phere ; and the electric discharge in such tubes presents 
certain peculiarities not observed in ordinary vacua. 

Electric action on substances inclosed in such tubes, 
and on the glass itself, is increased in the ratio of the in- 
creased vacuum; since those substances receive the force 
of energy which, in lower vacua, is expended on the air. 

This increased action is shown by an increase in the 
light and heat developed in them, and in the attractive 



150 



ELEMENTS OF STATIC ELECTRICITY. 



force exerted on them, as shown when they are free to 
move. 

Fig. 45 shows such a tube, having a glass railway on 
which is placed a roller with mica vanes, and the 
electrodes so placed that the upper vanes are in line 
between them. When an electric current passes 
through the tube, these vanes, being at zero potential, 
are attracted by the higher potential of the positive 
electrode, producing a rotary movement of the roller 
from negative to positive ; the force being sufficient to 
move it up an incline. 




Fig. 46— Rotary Movement Reversed. 



Fig. 46 shows a tube in which a wheel with mica 
vanes is so mounted that its center is in line between 
the positive electrode, and the center of the negative. 
The negative electrode a b is cup-shaped, and its con- 
cave surface turned towards the positive : so that the 
lines of force may be brought to a focus, and concen- 
trated on the vanes. And between its center and the 
wheel is placed the mica screen c d. 

A magnet, #, is suspended above the tube, between 



ELECTRIC TRANSMISSION IN VACUA. 



151 



the screen and negative electrode, in such a manner that 
it can be turned so as to reverse the position of its poles. 

By this means the electric current may be attracted 
or repelled, so as to pass over or under the screen. 
When it passes over the screen, the upper vanes are 
attracted towards ihe positive electrode, producing 
rotation of the wheel in accord- 
ance with such movement : but 
when the position of the magnet 
is reversed,the current is repelled 
and passes under the screen, and 
the lower vanes are attracted, 
reversing the rotation. 

The glass in these tubes is 
usually of very low insulating 
power, much lower than that 
of air at the ordinary density. 
Hence the electric resistance in 
high vacua, being much greater 
than in the glass, electric move- 
ment takes place through the 
vacua and glass respectively, in 
the inverse ratio of the resist- 
ance of each. 

Fig. 47 represents a tube in 
which the negative electrode 
consists of a half cylinder of 
aluminium, supported, near the center of the tube, on a 
small glass tube, b; through which a copper wire ex- 
tends, connecting the aluminium with the platinum 
terminal below. 

The ends of both electrodes come near the walls of 
the tube; and when the electric charge passes, its prin- 




Fig. 47 — Glass Illuminated 



152 ELEMENTS OF STATIC ELECTRICITY. 

cipal effect is produced on the glass, which gives a brill- 
iant green light ; the illuminated surface terminating in 
points near the extremities of the negative electrode. 

The influence of induction on the walls of the tube; 
as well as the conductivity of the glass, is illustrated in 
Fig. 48 ; which represents a pear-shaped tube, having 
for its positive electrode an aluminium cross, 6, placed 
near its broad end; the negative electrode a being 
cup-shaped as in Fig. 46. This cross is hinged at bot- 
tom to the platinum terminal; so that, by a movement 
of the tube, it can easily be brought to a horizontal or 
a vertical position. 



Fig. 48— Inductive Action of Metal Screen. 

When the charge is passed through the tube, the 
cross, when vertical, as shown in the cut, exerts a strong 
inductive influence on the broad end of the tube, to the 
left ; over a space inclosed by lines extending over its 
edges, from the negative electrode : repelling electricity 
from this space, and screening it from the action of the 
negative electrode, which attracts electricity from the 
other parts of the tube, and from the surrounding air. 



ELECTRIC TRANSMISSION IN VACUA. 153 

Hence electric action within this space is neutralized ; 
producing the dark shadow c d shown on the broad 
end ; while the rest of the tube is illuminated. 
• When the screen is thrown down a luminous cross 
takes the place of the dark shadow: but this higher 
illumination soon fades, since electric action on this 
space is now the same as on the rest of the tube. 

If the tube be used again, after a period of rest, the 
shadow can be reproduced; but is never so strong as at 
first. This proves that the glass has been subjected to 
an electric strain, which has permanently lessened its 
insulating power. 

The illumination of the glass is due to its resistance ; 
just as the bright spark is due to the resistance of air 
at the ordinary density, and the faint glow, to the 
reduced resistance in vacuum. Hence, when electric 
action begins, after the screen is thrown down, the 
resistance being greater on the spot which was pro- 
tected by the screen, we have the bright cross where 
the dark one was : but when the electric strain has so 
affected the relations of the molecules to each other, as 
to lessen the resistance, this first bright glow ceases, and 
the illumination is the same as in the rest of the tube. 

This action on the glass, as shown in Figs. 47 and 48, 
is accompanied with heat as well as light ; the tube 
shown in Fig. 47 becoming intensely hot, at those 
points where the greatest electric energy is concen- 
trated. 

Fig. 49 represents a tube constructed to show this 
heating effect in a very striking manner. Its upper 
part is enlarged into a globular form : and, at the bot- 
tom, is the concave negative electrode, of aluminium, 
already described ; which is so placed that it brings 



154 



ELEMENTS OF STATIC ELECTRICITY. 



the lines of force to a focus on a piece of iridio-platinum, 
5, placed in the center of the globe. This, being a 
metal of high resistance, becomes white hot under the 
electric action ; glowing with intense brilliancy, and 
finally melting. 

The walls of the globe, being remote from the line 

between the electrodes, 
which is comparatively 
short, the glass is less af- 
fected than in the long 
narrow tubes: so that elec- 
tric action is chiefly con- 
centrated on the object at 
the center. 

Crookes attributes all 
these phenomena to the 
impact of the residual air 
molecules, which he desig- 
nates as "radiant matter"; 
and claims that the mole- 
cules move independently 
of each other, and are 
driven with such force 
against the glass and other 
objects, as to produce the 
various phenomena de- 
scribed. 

Gordon considers this theory reasonable, and elabo- 
rates it at considerable length : but it is not generally 
accepted ; and it is believed that the explanations here 
given will be found more in accordance with well estab- 
lished electric principles. 




Fig. 49— Heat Produced in High 
Vacua. 



CHAPTER XI. 

Electrometers. 

Progress in every department of science is largely 
dependent on exact measurement, since it is only by 
this means that we get an accurate knowledge of 
relative values. The thermometer enables us to in- 
vestigate the laws of heat; the barometer gives us a 
knowledge of atmospheric pressure, and the various 
matters relating to it. And in chemistry and astron- 
omy almost every step depends on such measurement. 
Even our ordinary business transactions, and the value 
of our currency, are regulated by the common scales, 
by which we measure the force of gravity. 

Electric science is no exception to this rule. We 
require to know, accurately, relative differences of 
potential; the conductivity and resistance of various 
substances ; the force of electric attraction and repul- 
sion, the comparative energy of the various instruments 
used for generating and accumulating electricity ; and 
other matters of similar importance. 

But electric measurement presents peculiar difficul- 
ties not met with in the measurement of other forms 
of energy. In the measurement of gravity, we deal 
with a force easily controlled, the direction of whose 
movement is always known, and which, on the various 
parts of the earth's surface, is subject to but slight 
variation. 



156 



ELEMENTS OF STATIC ELECTRICITY. 



In heat we have a force, susceptible of easy control ; 
its movement slow, and its direction easily ascertained. 
But electricity moves with the rapidity of thought; 
its direction is difficult to ascertain ; and it defies our 
utmost efforts at absolute control; so that the results 
of measurement, by our best constructed instruments, 
fall short of perfect accuracy. 

In static electricity less progress has been made in 
measurement than in other forms of electric energy, 
whose practical applications are more numerous. 

The electroscope, sometimes 
classed with electrometers, in- 
dicates the presence of an elec- 
tric charge, but cannot be said 
to measure it, except as such 
indication may show an in- 
crease or diminution of a light 
charge. Lane's unit jar may 
be considered an electrometer, 
and the methods of measure- 
ment by it, and by sparks 
from the Holtz and Topler 
machines, belong to the same 
class : but both methods are 
very inaccurate, and can be 
used only in special cases. 
Coulomb's Torsion Balance. — To Coulomb is 
due the credit of the first efforts at accuracy in elec- 
tric science ; and the torsion balance, which is still 
extensively used, was his invention and may properly 
be regarded as the first electrometer. 

It is represented by Fig. 50; and consists of a glass 
cylinder A A, to the top of which is attached, at the 




Fig. 50- 



-Coulomb's Torsion 
Balance. 



ELECTROMETERS, 157 

center, a glass tube D D, to each end of which is fitted 
a brass collar. An enlarged section of the upper end 
of this tube and its attachments, representing what is 
known as the torsion head, is shown separately; in 
which it will be noticed, that the brass collar a has 
fitted to it a cap b with a projecting rim ; on the 
upper surface of which is a graduated scale, of 360 
equal divisions. This cap is capable of being turned 
horizontally, so as to bring the several divisions of the 
scale under a pointer <?, attached to a. 

In the center of b is a close fitting brass rod cZ, 
with a broad head by which it can be turned, when b 
is held firmly ; or the rod may be allowed to turn with 
b. Attached to this rod is a fine wire, which sustains, 
at its lower extremity, a horizontal shellac rod /, 
carrying at one end a small gilt ball g. Opposite this 
ball, on the cylinder A A, is a graduated scale a a, 
having 360 divisions, to correspond to those of the 
upper scale. Opposite the zero of this scale is a gilt 
ball g\ of the same size as the other gilt ball, and sup- 
ported by a shellac rod f n ', by which the ball can be 
introduced through an opening in the top of A A. 

The instrument is supported on a base, having level- 
ing screws ; and the air, in the interior, kept dry with 
chloride of calcium. 

To use this instrument, the cap b is turned till the 
zero of the upper scale is brought under the pointer c. 
The rod d is then turned till the movable ball g just 
touches the fixed ball g\ without torsion of the wire. 
The zeros of the two scales will then be practically in 
the same vertical plane. 

The fixed ball <f is then taken out, electrified, and 
replaced as before, in contact with the movable ball g. 



158 ELEMENTS OF STATIC ELECTRICITY. 

Both being the same size, the charge is equally divided 
between them ; and, being at the same potential, the 
movable ball g is repelled to a distance indicated by 
the number on the lower scale : at which point the 
force of repulsion is balanced by the torsion of the wire. 

The cap b is then turned in opposition to the repul- 
sion, so as to bring the ball g nearer to g'; the distance 
being indicated on the upper scale. The torsion of the 
wire is thus increased, and repulsion again balanced by 
torsion in the new position. 

It is known that the force of torsion is proportional 
to the angle of torsion : and since this force has to be 
increased to oppose the increase of repulsion, as the 
balls are brought closer, the point to be determined is 
the ratio of increase of force, as compared with the 
reduction of distance between the balls; which is done 
by comparing the distances from zero indicated on the 
upper and lower scales. 

The following is one of Coulomb's experiments for 
this purpose : The first distance to which the movable 
ball g was repelled being 36°, it was found necessary, 
in order to reduce this distance to 18°, to turn the cap 
b through 126°; and to reduce the distance to 8J° re- 
quired an additional rotation of the cap through 441°. 

The distances 36°, 18°, and 8i° are to each other, 
practically, in the ratio of 1, £, and I ; and the forces 
of repulsion at these points were balanced by torsions 
of 36°, of 126°+ 18°= 144°, and of 441° 4 126°+ 8J°= 
575i°, respectively. 

Now since 144 = 4x36, and 575J (practically 576) 
=16x36, it will be seen that as the distance between 
the balls is divided by 2 or by 4, the force of repulsion 
is multiplied by 4 or by 16 ; and thus Coulomb proved 



ELECTROMETERS 



159 



that electric repulsion varies inversely as the square of the 
distance. 

Inaccuracy of the Torsion Balance. — In the 
use of this instrument, as above, the arc is assumed as 
the distance between the balls, while the actual dis- 
tance is the chord of the arc ; but since these distances 
are in the same proportion, the accuracy of the results 
is not affected. 

It is also assumed that the arm of the lever, by which 
repulsion produces torsion, is the distance from the 
center of motion to the cen- 
ter of the ball g. But this 
is true only when the balls 
are in contact. In every 
other position, this arm is 
represented by a perpendic- 
ular from the center, on the 
chord which cuts the centers 
of the two balls : and as the 
ball g moves round, and the 
chord increases in length, 
this perpendicular decreases ; 
and vanishes when the chord 
equals the diameter. 

This is shown in Fig. 51, where b represents the first 
position of the balls, when the arm equals a b : but 
when g moves round to c, the line a f represents the 
arm ; and when it moves to d, the short line a m rep- 
resents the arm ; and at n the arm vanishes. 

This may be made more plain, by considering that 
the ball g is moving under the influence of two forces, 
electric repulsion, and the rigidity of the shellac rod, 
by which it is held at a fixed distance from the center. 




Fig. 51— Arm and Angle of 
Repulsion Illustrated. 



160 ELEMENTS OF STATIC ELECTRICITY. 

When motion begins, at 5, these forces act at right 
angles to each other but as the ball moves round, the 
angle of repulsion constantly decreases ; being repre- 
sented at e, by the angle a b c; and at cZ, by the 
angle a b d; and vanishing at ti, where the two forces 
are in direct opposition. 

In this position the force of repulsion opposes further 
movement : for, as it radiates equally in every direction 
from the two balls, its force on opposite sides of n is 
equal. But since, in the experiment given, the greatest 
angle was 36°, which is only one-fifth of the semi-circle, 
the error is not sufficient to affect the result seriously. 

Another inaccuracy results from lack of rigidity in 
the fine wire, which causes it to deviate slightly from a 
true vertical, under the influence of repulsion ; moving 
its lower extremity out of the center. 

There is also a slight inaccuracy resulting from the 
force of repulsion being estimated from the centers of 
the balls, instead of from their nearest points. 

It is also assumed that electric repulsion remains 
constant during the experiment : which would not be 
strictly true; since there is a continual reduction of 
energy, from causes already explained, which would 
produce serious error, if the experiment were of long 
duration. 

Since each ball becomes a center of electric radiation, 
it is evident that the lines of force cut by each repre- 
sent but a very small part of the entire repulsive energy. 
But since the balls are of equal size and equal poten- 
tial, it may be assumed that the proportion between 
the energy actually measured, and the entire energy, is 
the same in each ball. But an instrument embracing 
all the lines of force would evidently be more reliable. 



ELECTROMETERS. 161 

These inaccuracies doubtless account for the slight 
error observed in Coulomb's experiment, and tend to 
confirm the correctness of his results by showing suf- 
ficient cause for the error. 

Atteacted-Disc Electeometees. — Sir W. Snow 
Harris was the first to construct an electrometer on the 
attracted-ciisc principle. His instrument consisted of a 
scale beam, carrying at one end a pan for the weights, 
balanced at the other end by an insulated metal disc, sus- 
pended horizontally over a similar fixed, insulated disc. 

An electric charge being given to the lower disc, the 
force of attraction between it and the upper disc was 
measured by weights placed in the scale pan. 

The rapid loss of charge, from the edge of the elec- 
trified disc, was the chief objection to this instrument. 
But the principle has been adopted, and the construc- 
tion improved by Sir William Thomson, whose instru- 
ment, shown by Fig. 52, is described as follows: — 

Thomson's Absolute Electeometee. — This in- 
strument consists of two distinct parts ; one for testing 
and maintaining a certain constant auxiliary potential 
V, and the other for determining, in absolute measure, 
the difference between the potentials of any two given 
conductors. The first of these parts embraced a Ley- 
den jar, forming the case of the instrument, an idio- 
static gauge, and a replenisher E. 

The Levden jar is a glass cylinder, closed at top and 
bottom by metal plates: and coated, inside and out, 
with tin-foil, in which openings are left for viewing the 
internal parts; and an uncoated surface, for insulation, 
left at the top and bottom, between the inner coating 
and the metal plates. 

The idiostatic gauge will be understood from Fig. 



162 ELEMENTS OF STATIC ELECTRICITY. 




Fjcr. 52— Thomson's Absolute Electrometer. 



ELECTROMETERS. 



163 



53. A small aluminium plate P is fitted to a square 
hole in the metal plate G-, like a trap-door, without 
touching the edges. To one side of P is attached an 
arm A, of the same material, enlarged at its junction 
with P, and bent, so that when the surfaces of P and 
Gr are in the same plane, the arm is elevated a little 
above Gr, and is parallel with it. 

A platinum wire f stretched between tw^o supports, 
attached to Gr, passes through the enlarged part of the 
arm A, over a slight projection ; supporting P, and, by 
its torsion, regulating its movements. At the outer 
end of the arm is a fork 
F ; and between its 
prongs is a little white 
enameled standard, at- 
tached to Gr ; having, 
on its outer face, two 
black dots, close to- 
gether, and in the same 
vertical line. A black 
hair, stretched across the fork, and viewed through the 
lens ?, moves up and down in front of the dots ; and 
comes exactly between them, when the surfaces of P 
and Gr are in the same plane. This is called the sighted 
position. 

Under the plate Gr is seen, in Fig. 52, a circular 
metal plate P, supported on a metal rod, attached to 
the metal plate A, which is in contact with the inner 
coating of the Ley den jar; so that A and Pare always 
at the same potential F", as this coating. The distance 
between P and Gr is so regulated, that when the poten- 
tial of P is V, its attraction for the plate P overcomes 
the torsion of the platinum wire, and keeps P in the 




Fig. 53— The Idiostatic Gauge. 



164 



ELEMENTS OF STATIC ELECTRICITY. 



sighted position : and, in this way, the constancy of the 
potential Fis tested. 

This constancy of potential is maintained by the 
replenisher seen at E, which is practically a small To- 
pler machine ; and is shown separately in Fig. 54. A 
and B are two insulated metal inductors, to which are 
attached two receiver springs a and b. C and D are 
two contact springs, in electric connection with each 
other, but insulated from the other parts. 

P and Q are two metal 
carriers, attached to an eb- 
onite cross-piece, through 
which passes the ebonite 
axis T 7 , which can be ro- 
tated by the milled head 
E: so that the carriers 
P and Q, revolving inside 
the inductors A and B, 
shall successively touch 
the springs a, 2), 5, C. 

When the replenisher 
is in its place, as shown 
in Fig. 52, the inductor A is put in electric connection 
with the disc A ; which is supported in connection with 
the inner coating of the Leyden jar : while the inductor 
J9, being in contact with the cover, is in electric con- 
nection with the outer coating. And since the replen- 
isher operates on the principle of the Topler machine, 
already described, its rotation, either direct or reversed, 
will raise or lower the potential of the jar : and so keep 
the potential of the plate A, and of the idiostatic gauge, 
connected with it, at the constant potential V, as 
shown by the gauge. 




Fig. 54— The Replenisher. 



ELECTROMETERS. 165 

The second part of the electrometer consists of the 
apparatus for expressing differences of potential, be- 
tween conductors, in absolute measure. The metal 
plate A, called the guard plate, has, at its center, a 
circular opening about If inches in diameter, to which 
is fitted the disc C ; which just fills it without touching 
the edges; and is made of thin aluminium, flat and 
smooth on its under side, but strengthened by a rim, 
and radial arms, on its upper side. It is supported by 
three light steel springs, shaped somewhat like tuning- 
forks, and placed horizontally, at equal distances apart; 
one of which is shown at S. The lower end of each is 
•attached to the center of (7, and the upper end to a 
brass socket, which is cemented to the lower end of a 
glass rod, shown at I; which insulates it from the 
metal rod above; to the lower end of which the glass 
rod is attached. And the metal rod is moved vertically 
in guides by the micrometer screw M ; the movements 
being registered by the scale Gr, and the graduated disc 
D. 

To the center of the disc is attached a fine hair: 
in front of which a lens, IT, is so placed as to form, at 
its conjugate focus, near the surface of the jar, an 
image of the hair; which may be viewed through the 
eye-piece at L. This image is seen exactly between 
the points of two screws K y when the lower surfaces 
of the disc C, and guard plate A, are in the same plane : 
which is called the sighted position. 

On a support below J., is the metal disc B, known 
as the attracting disc ; insulated from the jar, and mov- 
able vertically by the micrometer screw M' ; the move- 
ments being registered by the scale E, and the grad- 
uated disc T. It is connected with the electrode N, by 



166 ELEMENTS OF STATIC ELECTRICITY. 

which it can be put in electric connection with bodies 
whose potential is to be tested. 

The attracted plates P and C are really movable 
centers of the guard plates Gr and A ; and since loss of 
charge, from radiation and otherwise, affects chiefly the 
outer edges, the small centers are practically unaffected 
by such loss. Hence the large discs Gr and A are 
appropriately called guard plates. 

Mode of Using the Absolute Electeometee. — 
The plates are first brought to zero potential, by put- 
ting them, for an instant, in electric connection, by the 
electrode JV", connecting with J9, and a wire connecting 
with A through the cover. The disc is then brought to 
its sighted position by the micrometer 31", and the read- 
ing noted. A known weight, w, is then placed upon it 
so as to depress it below the level of the guard plate 
A; and M is turned till is again raised to its sighted 
position : the reading is noted, and the weight removed. 

The Leyden jar is then charged to potential V, as 
determined by the idiostatic gauge, and kept constant 
by the replenishes during the experiment. The disc 
B is now put into connection with the outside coating 
by the electrode N; and the micrometer M f turned till 
the attraction of B on the disc C brings it again to its 
sighted position. Hence the attraction of B is known to 
be equal to the weight w. This reading being noted, B 
is insulated, and the bodies, the difference of whose po- 
tentials x and z is required, are successively put into 
contact with B through A 7 ". The distances d and h 
through which B has to be moved to bring the disc (7, 
in each case, to its sighted position, are noted, and the 
difference of potential can then be calculated. 

Formulae foe Charged Surfaces. — With a 



ELECTROMETERS. 167 

given charge, the electric energy at any point on a con- 
ductor, called its surface density, is in proportion to its 
surface area. Let q represent the surface density, then 
the electric force, exerted by a charged conductor on a 
point near it, equals q multiplied by the surface area. 

On a sphere the surface equals the square of its ra- 
dius multiplied by 4x3.14159. If 3.14159 = *, and 
radius = 1, we have I 2 x 4 it = 4 n. Hence the force 
exerted by a charged sphere on a point near it equals 
4 n q ; and the force exerted by a charged hemispher- 
ical surface equals 2 n q. 

The hemispherical surface may be considered as 
made up of the bases of an infinite number of small 
cones, having their apexes at the center. Hence each 
base subtends a solid angle : and lines of force, extend- 
ing from surface to center, are everywhere normal to 
the surface. 

Now if we conceive a plane surface applied to the 
hemispherical surface, and these cones extended to 
meet it ; we find that the lines of force, extending from 
these bases, are oblique to the plane surface. Hence 
each one can be resolved into, two components, one 
normal to the plane, and the other acting along it. 
But since there are an infinite number of these cones, 
the lines of force from whose bases may all be resolved 
in this way; the components along the plane, all 
around, neutralize each other, leaving only the normal 
components; whose force equals the sum of all the 
solid angles multiplied by the surface density, which, as 
we have seen, equals 2 no. Hence the expression is 
the same for a plane or a hemispherical surface. 

Application of Formulae to Measurements 
by Electrometer. — When there are two discs, at 



168 ELEMENTS OF STATIC ELECTRICITY. 

different potentials, near each other, as A and B in the 
electrometer, the attraction of each for the other is 
equal; the air being the dielectric between them. 
Hence the force, exerted at any point between them, 
equals the force on both surfaces, represented b) r 4?r(>; 
and tends to draw the movable disc C towards B. 
But this force is also equal to the difference of poten- 
tial, divided by the distance between the discs. Hence 

when x represents difference of potential, and d the 

/*» 
distance, the resultant force, at any point, equals — . 

xx 
Hence 4c7tp=— , and Q = - t — ;. 
a \7ta 

Now if the surface of the movable disc C be repre- 
sented by 8, its attractive force will equal s q : hence 

the total attractive force equals 27tQXSQ = 27ts() 2 . 

x 
And substituting for q its value, A ' Y , we have 
° 4 Tt a 

2 7ts( _) = 2ns » ™ = q — r 2 - 

V4 rt d' 16 7i" d" 8 *r cr 

Now since the attractive force equals the weight w, 

multiplied by the acceleration produced by gravity, 

represented by g, we have w g = ^ — — : therefore x 



8 7t d 2 



— d — w 9 (1), which expresses x in absolute meas- 
ure. But x represents the potential of the first body 
tested by the electrometer. 

By a similar process the potential, z, of the second 



body is expressed by the equation, z = h\ (2). 

Subtracting (2) from (1), we have x — z = (d—K) 

\%7twg 



ELECTROMETERS. 169 

By substituting figures for the letters in the second 

member of this equation, the difference of potential, of 

any two bodies we wish to test, may be expressed 

arithmetically. ,~ — — — 

8 it w q . 

The expression J is constant ; since it rep- 
resents the attraction of the disc B for C, when the 
Ley den jar is at the constant potential, V : while the 
expression (d — K) is variable; representing the differ- 
ence of distance, required by the variable difference of 
potential, expressed by x — z. 

Thomson's Quadrant Electbcmeter. — This in- 
strument, invented by Sir William Thomson, is highly 
esteemed for its great sensitiveness. It is represented 
by Fig. 55, and consists of a frame supporting a Leyden 
jar, which resembles an inverted glass shade, with a 
brass cover, to which the principal parts are attached. 

These consist of the idiostatic gauge and replenisher, 
already described, and the quadrants and needle, and 
parts connected with them. 

The jar contains strong sulphuric acid : which forms 
the inner coating, keeps the interior free from moist- 
ure, and forms a perfect connection with the needle, 
without friction. The outer coating consists of strips 
of tin-foil, connected with the cover and supporting 
frame. The upper part of the jar incloses the quad- 
rants and needle ; protecting the needle from currents 
of air, and permitting its movements to be'seen. 

Fig. 56 is an enlarged view of the needle and quad- 
rants. The needle is a thin, ilat piece of aluminium, 
shaped like a figure 8 ; represented by the dotted lines 
in Fig. 56 ; and seen edgewise in its place at w, in Fig. 
55. Through its center passes a piece of stout platinum 
wire to which it is attached, and which terminates 



170 



ELEMENTS OF STATIC ELECTRICITY. 



above in a small, T-shaped piece of metal: to which 
are attached, at the extremities of the cross piece, two 
fibers of unspun silk ; by which the needle is suspended 




Fig. 55— Thomson's Quadrant Electrometer. 



from a projecting arm, supported, in the upper part of 
the instrument, on a vertical glass rod. When the 
needle is at rest, in the fixed position between the 
quadrants, as shown in Fig. 56, the silk fibers hang 
parallel to each other, and the cross piece, below, is 



ELECTROMETERS. 



171 



then parallel to the projecting arm above. But in any 
other position, each fiber is at an angle with its vertical 
position, . and the needle slightly elevated : conse- 
quently the force of gravity tends constantly to turn 
the needle, without friction, back to its fixed position. 
This mode of suspension is termed bifilar. ' 

A platinum weight, suspended in the sulphuric acid 
by a fine platinum wire, from the lower end of the stiff 
wire below the needle, keeps 
the needle in position, and in 
contact with the inner coat- 
ing. 

The wire, above and below 
the needle, is inclosed in fixed 
guard tubes ; the lower one 
shown at iv : which screen it 
from external electric influ- 
ence; and furnish a connec- 
tion, by which the charge is given to the inner coating. 

The needle is inclosed within four brass quadrants : 
which, if joined, would form a circular box. They are 
separated from each other, and from the needle, as 
shown in Fig. 56 : and opposite pairs, A and A\ B and 
B\ are connected by fine wires ; and all supported at 
the same level ; and insulated, by glass rods attached 
to the cover. 

Three of them are permanently attached, but the 
fourth can be moved in and out horizontally ; guides, 
and a spring and counteracting screw, being arranged 
to keep it in position, and regulate its movement. 

Above the needle, and attached to its supporting 
wire, is a small concave mirror t ; by which a ray of 
light is reflected on a scale, placed in front of it, at a 




Fig. 56 — Quadrants and Needle. 



172 



ELEMENTS OF STATIC ELECTRICITY. 



distance of about 36 inches. This scale is shown in 
Fig. 57. Behind it is a lamp, the light from which 
conies through a vertical slit in a screen : above which 
is a horizontal screen, which cuts off the direct rays 
from below ; while the angle of reflection brings the 
ray from the mirror directly on the scale, where it 
appears as a small spot of light. Another screen, 
placed at an angle, 
cuts off the direct 
raj's from above. 

As the mirror turns 
with the needle, the 
reflected ray becomes 
a long pointer ; mov- 
ing without friction : 
by which the slight- 
est movement of the 
needle is indicated on 
the scale. 

In the instruments 
first constructed, the needle was suspended by a single 
fiber of silk ; and a small magnet attached to the back 
of the mirror: which, by the attraction between it and 
a large magnet, placed outside the jar, as shown in Fig. 
55, controlled and limited the movements of the needle ; 
the attraction of the magnets tending constantly to 
bring it back to its fixed position, where the spot of 
light rests on the zero of the scale. But the bifilar sus- 
pension is now preferred ; rendering the use of magnets 
unnecessary. 

At I and ?n, Fig. 55, are seen the chief electrodes; 
used to connect the opposite pairs of quadrants with 
bodies whose potential is to be tested : and at p is the 




Fig. 57 — Scale, Lamp, and Screen. 



ELECTROMETERS. 173 

charging electrode, used to connect the replenisher with 
the inner coating of the Leyclen jar. One pair of quad- 
rants, A A f , Fig. 56, is connected with the electrode ?, 
and the other pair, B B\ with the electrode m. 

Mode of Usixg the Quadraxt Electrometer. 
— The Leyden jar is connected with the replenisher by 
the electrode p, and charged to a certain constant po- 
tential, F, as indicated by the gauge ; and its constancy 
maintained during the experiment: and the needle, 
being connected with its inner coating, has therefore 
the same constant potential V. 

By means of the electrodes I and m, a connection is 
then made between the opposite pairs of quadrants, and 
any two bodies whose difference of potential is required; 
one of which is usually the earth. Suppose the earth 
connection to be made with the electrode m ; then, if 
the potential of the other body is higher than that of 
the earth, the needle will move round from the higher 
to the lower potential ; that is, from A A r to B B f : but 
if it is lower, the movement will be from B B r to A A!: 
and the difference of potential will be indicated on the 
scale by the movement of the spot of light, to the right 
or left from zero ; and may be considered practically 
correct, within certain limits. In this way the re- 
quired potentials are compared with the constant poten- 
tial V; and the results determined in absolute measure. 

In the Helmholtz quadrant electrometer the quad- 
rants are maintained at the constant potential; and the 
bodies whose potential is required are connected with 
the needle. 

There are various styles of Thomson's electrometers : 
both of the attracted-disc and quadrant instruments. 
Some of them are portable, and much simpler than 



174 ELEMENTS OF STATIC ELECTRICITY. 

those already described; the replenisher, gauge, and 
Leyden jar, being omitted; also the bifilar attachment 
in the quadrant instrument; the movements of the 
needle being controlled by the torsion of a fine wire. 
And, in the attracted-disc electrometer, the position of 
the discs is sometimes reversed; the attracting disc 
being placed above, in the portable style. 



CHAPTER XII. 
The Electricity of the Earth and Atmosphere. 



Potential and Earth Currents. 

Terrestrial and atmospheric electricity are so in- 
timately related, that to obtain a correct knowledge of 
either requires the consideration of its relations to the 
other. 

Viewing electricity as a universal property of mat- 
ter, its existence in the earth and atmosphere follows 
as a necessary consequence. Hence, we are to study 
its phenomena in this connection, rather than to ac- 
count for its origin. These phenomena pertain chiefly 
to difference of potential between different parts of the 
earth's surface; different parts of the atmosphere; and 
between the earth's surface and the atmosphere. 

This difference of potential results from various 
causes. We have already seen how difference of po- 
tential may be produced, artificially, bj^ various instru- 
ments, which are combinations of different substances, 
having different degrees of electric resistance and con- 
ductivity. By similar methods nature, on a grand 
scale, produces results of which ours are but feeble im- 
itations. 

Illustrations from the Thermopile. — In the 
thermopile we have an illustration of the method by 
which difference of potential is produced by heat. This 



176 ELEMENTS OF STATIC ELECTRICITY. 

instrument is a combination of metal bars, whose con- 
ductivity for heat and electricity varies greatly. A 
number of these bars, arranged in compact form, and 
proper]} 7- insulated, are soldered together in an alter- 
nating series: so that a current of electricity, passing 
through them, has to pass from one metal to the other. 
They are folded together, and mounted in such a man- 
ner, that heat may be applied to one set of junctions; 
while the opposite, alternate set, is cooled. 

In this waj^, instruments are constructed, in which a 
very slight difference of temperature, between the op- 
posite sets of junctions, creates a perceptible difference 
of electric potential: and powerful batteries are con- 
structed in the same manner. 

The earth may be regarded as an immense battery of 
this kind; being composed of heterogeneous materials, 
whose conductivity for heat and electricity varies 
greatly: raid which are subjected to great extremes of 
temperature, at opposite junctions, fulfilling exactly 
the conditions of the thermopile. 

The ocean, a vast, homogeneous conductor, is sepa- 
rated into different parts by the great continents; whose 
conductivity differs from it greatly: the five great 
divisions of the ocean, and the two continents, con- 
stituting an alternating series of conductors, of differ- 
ent conductivities. 

The surface of the continents, composed of rock and 
soil, of lakes, rivers, and sandy deserts, presents a great 
diversity of material, of widely different conductivity. 

In the torrid and frigid zones, we have the opposite 
extremes of temperature; which, in the thermo-electric 
battery, are produced by exposing one set of junctions 
to the heat of a lamp furnace ; while the opposite set is 



POTENTIAL AND EARTH CURRENTS. 177 

cooled with ice. Similarly also in the diurnal revolu- 
tion of the earth, opposite sides are subjected daily to a 
constantly changing temperature. And, in its annual 
revolution, we have the same result in the changing sea- 
sons; which also produce great changes in the conduct- 
ing character of the surface ; from the frozen, snow- 
clad surface of winter, to the verdure-clacl surface of 
summer. 

Diurnal and Seasonal Variation. — The change 
of electric potential produced by these causes in the 
earth, induces the opposite potential in the atmosphere ; 
which, by its lower strata, is insulated from it. Hence, 
in observations made on the potential of the earth and 
atmosphere, we find, as we should be led to expect, 
daily maxima and minima potential, and also seasonal 
maxima and minima. 

In several series of observations, made by different 
observers in Europe, both on the continent and in the 
British Isles, these maxima and minima were carefully 
noted: and it was found, that, in winter, the daily 
maxima occur at about 10 A. m. and 7 P. M.; in sum- 
mer at about 8 A. m. and 10 P. m.; and in spring and 
autumn, at about 9 A. M. and 9 P. M. The daily min- 
ima occur, in summer, at about 3 P.M., and midnight; 
but the daily winter minima are not given with suf- 
ficient definiteness to be reliable. 

From this we see, that the daily maxima, occurring 
soon after sunrise and sunset, correspond to the hours 
of greatest change of temperature ; while the daily 
minima occur at the hours when temperature is most 
constant. 

The seasonal maximum occurs in winter, and the 
seasonal minimum in summer: the maximum about 

12 



178 ELEMENTS OF STATIC ELECTRICITY. 

January, and the minimum in May and June. They 
are doubtless due, in part, to the different conduct- 
ivity of the earth's surface in summer and winter, 
as already mentioned ; and also to the dry winter 
atmosphere, when atmospheric insulation is high, as 
compared with the damp atmosphere of spring and 
early summer, when it is low; the greatest minimum 
occurring in the months when our atmosphere, in the 
north temperate zone, is most heavily laden with vapor. 
At this season the earth is covered with green, suc- 
culent herbage ; wet with frequent showers, and laden 
at night with heavy dews ; forming a conducting sur- 
face, which offers but slight resistance to electric trans- 
mission. 

Towards the close of summer, the grain ripens, the 
showers become less frequent, the dews lighter, and a 
vast expanse of dry straw and stubble, with a parched 
soil beneath it, offering high electric resistance, takes 
the place of the former conducting surface. As fall ad- 
vances, and the grass becomes dry and withered, and 
the trees shed their leaves, there is a constant increase 
of this surface resistance, and a corresponding increase 
of electric potential, till the winter maximum is reached. 

While this difference of conductivity in the land 
surface is taking place, the conductivity of the water 
surface remains practically constant : hence the period 
of minimum potential corresponds to that in which the 
difference of conductivity, between the land and water 
surfaces, is least; while the period of maximum poten- 
tial corresponds to that in which it is greatest ; point- 
ing clearly to this difference as a probable cause. 

In addition to the changes of electric potential, in- 
duced in the atmosphere by these changes in the elec- 



POTENTIAL AND EARTH CURRENTS. 179 

trie condition of the earth's surface, its electricity is 
doubtless affected, directly, by conditions similar to 
those which affect the earth's electricity. 

In its combination of dry air and watery vapor ; the 
one, an insulator, and the other, a conductor ; separate 
parts heated and cooled alternately, twice in twenty- 
four hours, we have thermo-electric conditions similar to 
those already noticed in the earth's surface ; though 
the resulting electric disturbance is, perhaps, less in- 
tense, as the composition of the atmosphere is nearly 
uniform, while that of the earth's surface presents great 
diversity. 

Difference of Potential between Atmos- 
pheric Strata. — Another cause of atmospheric elec- 
tric disturbance is found in the great difference of 
electric resistance between the upper and lower atmos- 
pheric strata; caused by the density below and rarity 
above. This resistance makes the dense lower stratum, 
where most of our observations are made, an excellent 
insulator ; while the rarity of higher strata facilitates 
electric transmission; a constant decrease of resistance 
taking place, from the lower to the higher, till a point 
is reached, where it is reduced to that of the ordinary 
Geissler tube ; while, in still higher strata, the resist- 
ance increases, on account of the extreme rarity of the 
air: which equals, and finally exceeds, that of the best 
vacuum tubes. 

The existence of a corresponding difference of elec- 
tric potential has been proved by numerous experi- 
ments ; among which may be noted the following: — 

From an elevated position, a metal -pointed arrow 
was shot upward to a vertical height of 250 feet: a con- 
ducting cord, connected with it, and properly insulated, 



180 ELEMENTS OF STATIC ELECTRICITY. 

communicated with an electroscope at its lower extrem- 
ity. As the arrow rose, the electroscope showed a 
steadily increasing difference of potential, till the 
indications equaled the full capacity of the instrument. 

The arrow was then shot horizontally, at an eleva- 
tion of about three feet, but no change of potential was 
indicated; proving' that the indications resulted from a 
difference of potential existing in the atmosphere, and 
were not due to the friction of the arrow in passing 
through the air. 

The difference of potential, in this experiment, was 
between the earth and atmosphere : but the following 
experiment was entirely independent of the earth. 
During a balloon ascent, a conductor, 170 feet in length, 
was lowered into the air ; a ball being attached to its 
lower end, and its upper end connected with an elec- 
troscope. The indications showed a marked difference 
of potential between the upper and lower strata. 

As the balloon moved with the wind, the friction 
between the ball and the air could not have been suf- 
ficient to affect the electroscope perceptibly ; so that, 
in this instance, as in the former, the indications of the 
instrument must be attributed to a difference of po- 
tential existing in the atmosphere. 

The series of observations already referred to, and 
numerous others of a similar character, prove that the 
potential of the atmosphere is almost invariably positive 
with reference to that of the earth. 

The Atmosphere as a Leydex Jar. — It is evi- 
dent that we have, in the atmosphere and on the earth's 
surface, the same conditions which exist in the Leyden 
jar — two conducting surfaces insulated by a dielectric ; 
the stratum of least resistance forming the upper con- 



POTENTIAL AND EARTH CURRENTS. 181 

ducting surface; the earth's surface, the lower one; and 
the dense lower stratum, the dielectric. And, as in 
the Leyden jar, any change of potential in either sur- 
face produces the opposite electric condition in the 
other surface. 

The upper surface, being insulated, corresponds to 
the inner coating; and the lower uninsulated surface, 
to the outer coating. But since those surfaces are of 
vast extent, any limited area of upper surface would be 
connected with a conducting surface at its outer edges; 
through which connection electricity would be repelled 
from this area, or attracted to it, as the potential of the 
surface below it had a greater or less intensity. But 
the earth connection, of the lower surface, would be 
exactly the same as that* of the outer coating of the 
Leyden jar. 

We live and move on the outer coating of this Ley- 
den jar; on a surface practically equipotential within 
limited areas ; and hence do not perceive electric action 
taking place, no matter how highly charged the jar 
may be, except when the tension becomes strong 
enough to overcome the resistance of the dielectric, or 
to render prominent or visible the action on either 
side of it. 

This surface then, which we call neutral, is really a 
charged surface ; but, like the outer coating of a 
charged Leyden jar, quiescent, till brought into action 
by connection with the inner coating, or by induction 
between the two. 

ASCENDING AND DESCENDING CUEUEXTS. — We 

have seen how air currents are produced by the action 
of an electric machine, and how light bodies vibrate 
between electrodes connected with opposite surfaces of 



182 ELEMENTS OF STATIC ELECTRICITY. 

a charged Leyden jar. Now since a constant difference 
of potential is proved to exist between the earth's 
surface and the atmosphere, and between upper and 
lower atmospheric strata, we must conclude that 
ascending and descending currents result from this 
difference : and that the clouds, and the invisible vapor 
diffused through the air, are, like the air, subject to this 
constant electric movement. But, there being also a 
horizontal movement, due to the winds, the resultant 
of the two movements is a series of curves, ascending 
and descending, as the body of air and vapor moves 
over areas of high or low potential. 

The air and vapor in contact with the earth, becom- 
ing electrified to the same potential as the earth's sur- 
face, are repelled, and attracted upward by the force 
resulting from difference of potential in the stratum of 
least resistance above. Similarly the air and vapor 
above are repelled, and attracted downward in conse- 
quence of the difference of potential below. 

The morning and evening maxima, occurring at 
opposite points in the rational horizon, show that two 
electric waves traverse the surface daily from east to 
west, as the earth revolves from west to east. And, at 
points about equally distant from these waves, follow 
the two daily minima. During the maxima the ascend- 
ing and descending currents must acquire a great 
increase, both in volume and in acceleration of move- 
ment: while the minima, preceding and following, 
create horizontal movements between the areas of high 
and low potential ; producing resultant curves, similar 
to those due to the winds, but recurring in regular 
succession. In fact these currents are themselves 
electric winds. 



POTENTIAL AND EARTH CURRENTS. 183 

The rarefying of the air from heat, at the time of the 
morning maximum, must increase and accelerate the as- 
cending current, while its condensation from cold, at the 
evening maximum, similarly affects the descending cur- 
rent; gravity in each case supplementing electric force. 

Cosmic Electric Influence. — Assuming that 
electricity is a universal force, acting through matter 
in different forms, as a universal medium, it follows 
that electric induction is universal. Hence induction 
between our planet and the other members of the solar 
system, especially the sun and moon, must affect the 
electric condition of the earth and atmosphere. 

It is considered a well established principle, that the 
tides are due to the attraction of the sun and moon, 
attributed to gravity. But the daily electric maxima 
and minima indicate that there are electric tides, coin- 
cident with the ocean tides, due to the electric induc- 
tion of the sun and moon : that an electric impulse 
follows the earth's movement, as different portions of 
its surface are successively exposed to this influence 
during its daily rotation, producing electric currents 
in both the land and water surface ; and perhaps also 
tidal waves in the ocean and atmosphere. 

We have seen that when a charged sphere is placed 
near the end of a cylinder, or of the longer axis of a 
spheroid, the electricity of the cylinder or spheroid is 
either repelled or attracted by induction, according as 
the potential of the sphere is positive or negative, w r ith 
reference to that of the other body ; and that this effect 
is intensified when two charged spheres, at different 
potentials, are placed at opposite ends of the cylinder, 
or longer axis of the spheroid. If both are placed at 
the same end, the inductive effect is a mean between 



184 ELEMENTS OF STATIC ELECTRICITY. 

the two effects. But if one be placed opposite the 
center of the c}~linder or spheroid, so that its action is 
at right angles to that of the other, the intensity of 
action at the ends is diminished. 

In the sun, moon, and earth, these conditions are 
exactly fulfilled as to shape and position ; and, prob- 
ably also, as to difference of potential. The earth is 
an oblate spheroid, whose longer axis lies east and west ; 
pointing nearly to the apparent path of the sun and 
moon. Hence at the full moon, the new moon, and 
the quarters, we must have the same inductive effects 
as in the experiment with the spheroid and the two 
spheres. The earth, at full moon, is between the sun 
and moon, and receives the highest inductive effect. 
At new moon they are on the same side of it, and 
nearly in line, and their effect, if at different potentials, 
is lessened : while, at the quarters, when the induction 
from each is at right angles to that of the other, it is at 
its minimum. Hence we should expect to find, as in the 
ocean tides, electric neap and spring, ebb and flood tides. 

Very little is known of the relative inductive influ- 
ence of the sun and moon on the earth. Judging from 
the analogy of the ocean tides, we might infer that the 
induction of the moon is greatly in excess of that of 
the sun. But in estimating effects produced by gravity, 
the two principal factors are mass and square of dis- 
tance ; whereas, in estimating inductive electric effects, 
the various agencies by which electricity is generated 
must also be taken into account. 

The nearness of the moon to the earth causes its 
effect on the ocean tides to be much greater than that 
of the sun, though its mass, as compared with the mass 
of the sun, is only as 1 to 26,400,000. But, in consid- 



POTENTIAL AND EARTH CURRENTS. 185 

ering the electric influence of the two bodies, we find 
that the lunar surface is that of a dead world, abso- 
lutely quiescent, so far as we know; while the solar 
surface, to a great depth, is in a state of the most 
violent agitation. From which we must infer a great 
difference of electric potential in favor of the sun. 
And observation indicates that this state of agitation 
affects the earth's electricity ; while we have no obser- 
vations of electric effects produced by the moon. 

Certain electric phenomena on the earth are found 
to coincide with certain solar phenomena. These con- 
sist in violent oscillations of the magnetic needle 
during prominent solar disturbances, indicated by the 
sun spots. And it is found that the periods of max- 
imum solar disturbance, which occur once in eleven 
years, are noted for corresponding maxima in those 
perturbations of the magnetic needle. 

To make this clear, it should be stated that magnet- 
ism is produced, artificially, by the circulation of an 
electric current around a conductor capable of being 
magnetized, at right angles to its length ; as by a 
current circulating in a coil of w r ire, round a bar of 
iron or steel. And, conversely, a magnet generates an 
electric current in such a coil. 

It is known that the earth is a great natural magnet; 
having north and south magnetic poles, which exercise 
a directive force on the magnetic needle ; and it seems 
highly probable, that its magnetism is the result of 
electric waves, or impulses, circulating round it from 
east to west as has been shown; giving rise to electric 
currents ; and due to difference of temperature, and to 
solar and lunar influences : and that the perturbations 
of the magnetic needle, coincident with solar disturb- 



186 ELEMENTS OF STATIC ELECTRICITY. 

ances, are the result of corresponding disturbances in 
these electric movements. 

Observations on Telegraph Lines. — The tel- 
egraph affords special facilities for observing many of 
the phenomena pertaining to terrestrial and atmospheric 
electricity, by means of its long lines of nearly uniform 
conductivity, insulated in the air, having earth con- 
nections at points remote from each other, and extend- 
ing, in the United States, chiefly, either at right angles 
to the magnetic meridian, or parallel with it. 

These facts have been recognized; and, within the 
last five years, observations have been made, on a 
limited scale, in the United States, and in Europe. 
These observations have been somewhat desultory and 
local ; no general, extended, well established system 
having yet been instituted. 

During the fall and winter of 1883-84, a series of 
observations was made on a line belonging to the 
Postal Telegraph Co.; extending, at first, from New 
York City to Meadville, Pa.; 509 miles by wire, 325 
direct; but subsequently completed to Chicago; 1058 
miles by wire, 725 direct. The observations from Oct. 
18 to Nov. 20, 1883, were between New York and Mead- 
ville; and the subsequent observations, which were con- 
tinued during November and December, 1883, and part 
of February, 1884, were between New York and Chicago. 

The line consisted of a large copper wire having a 
steel core ; thus combining conductivity and strength ; 
and the object of the observations was to ascertain the 
relations of the electric current to difference of temper- 
ature. They were made daily, at both ends of the line, 
at the hours when it was least occupied with other 
business, 8 to 8.30 a.m., 5 to 5.30 and 11 to 11 30 p.m. 



POTENTIAL AND EARTH CURRENTS. 187 

The line being disconnected from the batteries, and 
connected with the earth at both ends, the current was 
obtained from the earth alone, independent of any artifi- 
cial source: and its strength and direction, as indicated by 
the galvanometer, were noted, and also the temperature. 

It was found, that the general direction of the cur- 
rent was from a region of high to one of low temper- 
ature, though frequent reversals of current were 
observed. And as the east, from longer exposure to 
the sun's heat, would have a higher temperature than 
the west, at the time of the morning observation, the 
prevailing current, at this hour, was found to be from 
east to west. As these conditions of temperature 
would be reversed in the evening, the observations at 
that hour- showed a corresponding reversal, and a 
prevailing w 7 est to east current. While the observa- 
tions near midnight, when another reversal of temper- 
ature is at hand, showed that the current then was 
fluctuating and uncertain. 

The deflection of the galvanometer needle varied 
from to 57°; the morning average being 11. 4°, the 
evening average 14.3°, the average near midnight 7.3°, 
and the general average 11°. The difference of tem- 
perature, between the points of observation, varied 
from to 37°; the morning average being 14.5°, the 
evening average 9.5°, the average near midnight 10.3°, 
and the general average 11.4°. 

When the earth connection was severed, at either 
station, the current was reduced to a minimum ; cor- 
responding to the probable leakage along the line ; 
proving that it was an earth current, and not an 
atmospheric current. 

If a similar east and west line were extended round 



188 ELEMENTS OF STATIC ELECTRICITY. 

the globe, we may reasonably infer that similar results 
would be observed on every part of it: and hence, 
that east and west currents are constantly traversing 
the earth, as it revolves from west to east. 

This will be more fully understood, when we con- 
sider, that, during the diurnal revolution of the earth, 
the sun occupies practically a fixed position with ref- 
erence to it : so that from the earth's heated hemi- 
sphere, electric currents are constantly flowing, from a 
central point where the sun's rays are vertical, in 
opposite directions, towards a point within the cooler 
hemisphere, opposite to the sun. 

But the diurnal revolution of the earth brings any 
limited area of its surface, surrounding an observer, 
alternately into each of these currents. So that, while 
they have a fixed direction with reference to the sun, 
and to the earth, as a whole; they become, alternately, 
east or west currents, with reference to such an area. 

From noon to midnight this area would be in the 
west to east current ; and, from midnight to noon, in 
the east to west current; an equatorial point, on the 
observer's meridian, passing the point from which the 
currents diverge, at noon ; and reaching the point 
towards which they converge, at midnight. 

At both these hours, the temperature, at equally 
distant points in the observer's latitude, reaching from 
his position, east and west to the sensible horizon, is 
nearly the same : and the noon and midnight minima 
of electric potential are the result. 

At sunset and sunrise the temperature on similar 
quadrants of the observer's latitude, east and west of his 
position, attains its maximum difference; and the even- 
ing and morning maxima of electric potential occur. 



POTENTIAL AND EARTH CURRENTS. 189 

It will be observed that while the observer's position 
reaches the point of highest temperature at noon, the 
point of lowest temperature is reached at sunrise. For 
the heating of any given area begins at sunrise, 
increases till noon, as the sun's rays become more 
vertical; and declines from that hour till sunset, as 
the rays become less vertical; while the cooling is 
constant from sunset to sunrise. So that the morning 
difference of temperature, between east and west 
regions, is greater than the evening difference ; and we 
should expect to find a corresponding increase of elec- 
tric potential, at the morning maximum. 

But the series of telegraphic observations given 
shows the reverse; which may result from the fact that 
the line on which the observations were made, has the 
Atlantic ocean at its eastern terminus, and the interior 
of the continent at its western. And, as change of 
temperature is much slower on a water surface than on 
a land surface, the difference of temperature between the 
Atlantic on the east, receiving the sun's rays first, and the 
interior on the west, would be less in the morning than in 
the evening, when these relative positions are reversed. 

As the distance between heated and cooled regions 
alternately increases or diminishes during the earth's 
diurnal revolution, electric resistance increases or di- 
minishes in the same ratio, and increase or decrease of 
current intensity is a corresponding result : and electric 
maxima and minima, and also reversal of current, must 
follow from this cause, as well as from difference or 
equality of temperature. But as increase or decrease 
of distance is coincident with increase or decrease of 
difference of temperature, the two causes intensify each 
other's effects. 



CHAPTER XIII. 
The Electricity of the Earth and Atmosphere. 



The Aurora. 



The relations of the aurora to terrestrial and atmos- 
pheric electricity present a problem of the deepest inter- 
est and importance, whose satisfactory solution must 
render clear many questions now involved in doubt and 
obscurity. Hence, during the last fifty years, it has 
been carefully observed, and a number of important facts 
in regard to it ascertained. The laws which govern it 
are still far from being understood, and much con- 
flict of opinion exists in regard to many points; but its 
electric origin may be regarded as fully established. 

This phenomenon occurs in zones surrounding the 
northern and southern magnetic poles. And obser- 
vations have been chiefly confined to its occurrence in 
the north. The northern aurora is known as the 
aurora borealis, the southern as the aurora australis, 
while the term aurora polaris, or simply the aurora, is 
applied to either. 

In the United States it is usually first seen 
at from 8 to 10 P. M., though often beginning much 
later : and it continues from three to four hours. Its 
occurrence during the day, also, is probable ; though 
it can only be inferred from coincident effects; the 
brilliancy of the daylight rendering it invisible. 



THE AURORA. 191 

Some of the great auroras have been seen for several 
nights in succession ; their occurrence during the inter- 
vening days also being highly probable. 

Auroral Arches, Corona, and Streamers. — 
It first appears, usually, as a low arch of light, in the 
direction of the pole, resembling the dawn of day; 
whence its name, aurora, the morning. This arch is 
often accompanied by a low bank of clouds, lying 
under it, next the horizon. As the arch slowly rises 
streamers of light, differing in color, size, and brilliancy, 
dart up through it ; extending from the horizon to 
a considerable height above the arch ; their color 
varying from a pale white to a light red; though yel- 
low, green, and blue tints have also been observed ; 
the prevailing tints differing more or less in different 
localities. 

These streamers appear to radiate from a central 
region below the horizon, cutting the arch vertically, 
at right angles, as shown in Fig. 58. The streamers 
sometimes appear to rise from widely separated points 
in the horizon; and, as the aurora increases in size and 
brilliancy, they culminate at the zenith, as shown in 
Fig. 59, forming a corona of more or less prominence; 
one of the most prominent being shown in Fig. 60. 

By comparing the three cuts, it will be seen, that if 
the center of the corona shown in Fig. 59, or Fig. 60, 
were below the horizon, the appearance would be the 
same as in Fig. 58. So that, supposing the observer 
placed below the horizon, under the center from which 
the streamers seem to emanate, he would see the 
corona above him, as in Figs. 59 and 60. And, con- 
versely, an observer in the latitude of Paris, looking at 
the corona, observed in latitude 70° N., Fig. 60, would 




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THE AURORA, 193 

see only the upper part of its southern half, corre- 
sponding to the aurora shown in Fig. 58. 

In the aurora shown in Fig. 61, seen from the Vega, 
in latitude 65° N., we have an arch formation without 
streamers. A series of concentric arch segments, more 
or less perfect, is seen ; the outer one less than a semi- 
circle, and the most perfect of the inner ones greater 
than a semicircle ; the central one, a double arch, with 
the nucleus of a second double arch above the junc- 
tion. From an inspection of the figure, it is evident, 
that the perfect arches would appear as complete circu- 
lar belts to an observer under the central point near 
the horizon. 

Difference of longitude, as well as latitude, must also 
modify the appearance : as that portion of the arch 
which appears to one observer as its summit, appears 
to another, at a distant east or west point in the same 
latitude, as its east or west base. And, supposing the 
first observer placed in the magnetic meridian which 
coincides with the center of the aurora, the effect of 
perspective would cause it to assume a different appear- 
ance to him, from that seen by the other observer, 
viewing it from a different angle. A streamer, seen 
from one position, would appear foreshortened; while 
at a different angle it would appear elongated: to one 
observer it might appear as a narrow ray, to another as 
a broad band. 

Hence, we may infer, that we see in the arch, rising 
from the horizon, the outer edge of a circular belt 
of electric light, with its varied phenomena of arches, 
streamers, rays, and coronee, covering a large area, 
parallel to the earth's surface, and extending, as it 
increases in size, from a region surrounding the pole, 




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THE AURORA. 195 

towards the equator : and that its different aspects, at 
different times and places, and its different phases, as 
seen at the same time by observers at different points, 
are greatly modified by its position with reference to 
the position of the observer. 




Fig. 60 — Auroral Corona Observed at Bossekop, Lat. 70° N. 

Auroral Movement, Curtain Formation. — A 
peculiar feature of the aurora is the continual move- 
ment visible in every part. A streamer darts up 
rapidly from the horizon, increasing in size and brill- 
ianc}^; and as rapidly fades away. Along one part of 
the arch a series of streamers form in rapid succession, 
giving the impression of an undulatory, horizontal 
movement, at right angles to the vertical movement of 
the rising streamers : and, as the intensity of this phase 
decreases, a similar movement, at some distant point, 
rises and declines in a similar manner. At times there 
occurs a curtain formation, composed of parallel rays ; 
appearing either as a single curtain, as shown in Fig. 
62, or as a series of curtains, hung one behind the 
other, showing only their lower margins, as in Fig. 63; 




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THE AURORA. 



197 



undulatory movements occurring, transverse to the 
apparent vertical position of the rays, like the move- 
ments of a banner floating in the breeze. 

This appearance is doubtless greatly modified by 
perspective: the rays which are apparently vertical, 




Fig. 62 — Auroral Curtain Formation, Observed at Bossekop, Lat. 70° N. 

being horizontal ; and probably emanating from the 
edge of an arch; producing the single curtain shown in 
Fig. 62 ; or from the edges of several concentric arches, 
like those shown in Fig. 61 ; producing the series of 




Fig. 63— Auroral Curtain Formation, Observed at Bossekop, Lat. 70° N. 

curtains shown in Fig. 63. It is also evident from this, 
that in the formation of coronae, the appearance is 
probably often due to the edge of the arch, with the 
streamers emanating from it, reaching the zenith of 
the observer. 



198 ELEMENTS OF STATIC ELECTRICITY. 

Auroral Bands. — Sometimes a single streamer 
spans the heavens from west to east like a band. The 
author saw such a one at Chicago, Oct. 5, 1882. 
Appearing at about 10.30 p. M., near the horizon, a 
little north of west, it extended, within ten minutes, 
to the eastern horizon, passing near the zenith : and 
remained visible for more than half an hour. Its 
apparent width was about four degrees, and its color a 
light red. 

The signal service record, for the same date, de- 
scribes an aurora, " seen generally throughout New 
England, as far south as Washington, and, in the 
northwest, from 10 30 p. M. till after midnight ; reach- 
ing an altitude of 90°, and covering 90° of the horizon." 
Its different colors, in different localities, were " white, 
blue, yellow, and crimson. Beams, arches, waves, stream- 
ers, and patches of light were visible ; and, at Wash- 
ington, frequent flashes of lightning, at the edge of the 
dark segment." 

Height of the Aurora. — Great diversity of opin- 
ion has existed in regard to the height of the aurora 
above the earth. A great altitude has been assigned 
to it by some, who argue that the same aurora could 
not otherwise be visible to observers thousands of miles 
apart : while others assign to it a low altitude ; main- 
taining that these different observers do not see the same 
aurora, but different ones, occurring at the same time: 
since the appearance seen by one, often differs greatly 
from that seen by another. But, since different parts 
of the same aurora maj^ be visible to different observ- 
ers, it is evident, that one of low altitude, and great 
extent, might be seen at points as widely remote from 
each other as the eastern and western continents ; the 



THE AURORA. 199 

electrified stratum of the atmosphere surrounding the 
polar area, like a circular belt. 

The weight of evidence is now in favor of the low 
altitude ; sixty-nine miles above the surface being con- 
sidered a fair estimate. But strict accuracy is not 
attainable ; since it is impossible for any two observers, 
at opposite ends of a base line of sufficient length, to 
fix with certainty on the same point, so as to make an 
angular measurement. But we can estimate the prob- 
able height at which atmospheric resistance would be 
sufficiently reduced to produce the auroral phenomena ; 
and we have already seen that this plane of least 
resistance must lie between the dense strata below 
and the region of high vacuum above; both of which 
oppose electric movement. Hence the height, given 
above, may be approximately correct ; and yet subject, 
doubtless, to variation, resulting from difference of 
atmospheric pressure ; low pressure diminishing resist- 
ance, and depressing the auroral plane, and high pres- 
sure producing the opposite effect. 

Geographical Position of the Aurora. — Ob- 
servation shows that the aurora is confined to com- 
paratively narrow belts. It is never seen at the 
equator, and is rarely visible in the northern hemi- 
sphere south of latitude 40°: while in higher northern 
latitudes, it is seen to the south of the observer; and 
decreases in frequency and brilliancy, assuming appar- 
ently a more southerly position, as the observer moves 
farther north. 

In Fig. 6-1, we have a chart, giving the results of 
observations made in the northern hemisphere, by dif- 
ferent European observers; which shows that this 
auroral belt is about 30° in width. Its southern limit, 



200 ELEMENTS OF STATIC ELECTRICITY. 




Fig. 64 — Chart showing Isochasmen or Lines of Equal Auroral Frequency. 
(From Petermann's Mittheilungen, 20 Band, 1874— IX.) 



THE AURORA. 201 

in the western hemisphere, is shown at Lat. 22° N., Long. 
75° W. from Greenwich; and its northern limit, on 
the same meridian, at Lat. 58° N. In the eastern 
hemisphere, its southern and northern limits, on the 
same meridian, are between 47° N. and 77° N. 

The increased width and number of the lines, towards 
the northern limit, show a great increase in the fre- 
quency, brilliancy, and duration of the auroras in that 
region. 

It is also found, that the position of this auroral belt 
varies at different seasons of the year; reaching its 
southern limit near the equinoxes, and its northern 
limit near the solstices. 

The results given in the above chart must be regarded 
as approximate, rather than strictly accurate ; as the data 
on which they are based were more or less imperfect. 

Causes of the Aueora. — Having now examined 
the various phases of auroral phenomena, and their 
location, we are prepared to investigate more fully the 
causes by which they are produced. 

The earth has already been described as a thermo- 
electric battery, and the atmosphere as a Ley den jar; 
the one a generator and the other an accumulator; and, 
in the combination of the two, we may look for the 
principal cause of the aurora. 

We have seen that electric movement is from higher 
to lower temperature, producing earth currents on east 
and west lines, governed by the earth's rotation, and 
by solar and lunar influence. But the greater differ- 
ence of temperature between the equatorial and polar 
regions must produce north and south currents of far 
greater energy than these east and west currents. 

It has also been shown, that a change of potential, in 



202 ELEMENTS OF STATIC ELECTRICITY. 

any portion of the earth's surface, must produce a 
corresponding change in the stratum of least resistance, 
in the atmosphere above it; and that a transfer of 
electricity must occur between this electrified atmos- 
pheric area, and the surrounding atmosphere, lying in 
the same horizontal plane, either from it or to it, as the 
earth's surface below is positive or negative. 

We have, in the aurora, the exact fulfillment of all 
these conditions. A high earth potential, in the polar 
regions, must result from the currents flowing in from 
the warm region ; and produce, by induction, a corre- 
sponding negative potential in the atmosphere. And, 
in the belts where these ice-bound polar regions join 
the warmer region, the principal electric action must 
take place; producing the auroral arches of white light : 
while the electricity radiating in opposite directions, 
north and south from the arch, produces the streamers, 
beams, rays, bands, and coronae ; as the electric action 
at different points has greater or less intensity, or meets 
with varying resistance. 

Confirmatory evidence of this view is found in the 
fact, shown by the chart on page 200, that within the 
torrid and north frigid zones, where a comparatively 
even temperature exists, the aurora is not seen ; and 
also in the shifting position of the auroral belt with 
change of temperature, as already mentioned. 

The east and west earth currents must also exercise 
their inductive influence, giving rise, probably, to the 
transverse undulations observed in the streamers and 
curtain formations. And the resultants of these cur- 
rents, and the north and south currents, are seen in the 
bands and streamers which often assume a diagonal 
direction, northwest and southeast, or otherwise. 



THE AURORA. 203 

The stratum, in which these phenomena occur, must 
have a certain degree of thickness; its upper surface 
merging into the region of high vacuum, and its lower 
surface into that of greater density ; resistance increas- 
ing upwards and downwards from a central plane. 
Hence, different phases of electric action must occur at 
different altitudes; corresponding to the different aspects 
of electric transmission in high and low vacua, seen in 
laboratory experiments, as described in Chapter X : 
which may account for the common auroral appearance, 
shown in Fig. 58, where the arch seems to form a back- 
ground for the streamers. And, .as there is often a 
series of concentric arches, as shown in Fig. 61, it is 
easy to see how streamers might radiate from one arch, 
across the plane of another arch, at a different altitude. 
And, if one was below, and the other above the horizon, 
the appearance would be the same as in Fig. 58. 

Now, since the causes here assigned are in constant 
operation, we may infer that there should be a constant 
aurora : though it does not follow, that it should be 
everywhere constantly visible. And from the great 
number of auroras observed in the course of the year, 
in different parts of the auroral belts, especially in the 
northern part of the northern belt, it is reasonable to 
infer, that, with a more perfect system of observation, 
auroras, of greater or less magnitude, would be seen, at 
one or more points, every night in the year. 

It is also probable that this electric action may be 
constant, without being always sufficiently intense to 
attract attention : and that the aurora is the result of 
its increased intensity. 

Other atmospheric phenomena, not usually recognized 
as belonging to the aurora, may also be due to this 



204 ELEMENTS OF STATIC ELECTRICITY. 

electric action. The peculiar band and arch formation 
of cirro-stratus clouds often strongly resembling auroral 
bands and arches, has, by many observers, been attrib- 
uted to similar electric action ; though doubtless occur- 
ring at a much lower altitude than that of the aurora. 

The existence of strong earth currents during the 
prevalence of auroras, and of those violent perturba- 
tions, known as "electric storms," are well established 
facts, proved by observations on telegraph lines. Dur- 
ing the aurora of Feb. 4, 1872, visible over an area 
embracing 30° of latitude, and 150° of longitude, these 
currents and perturbations were observed on all the 
lines within this area, both land and submarine ; being 
strongest on those having a southeast and northwest 
direction. 

The following description of the auroral storm of 
Nov. 17, 1882, is condensed from the Signal Service 
Reports : " Beginning a little before daylight, it was 
known at first by its interference with telegraphy. 
For three hours not a wire of the Western Union Tel- 
egraph Company could be worked. Late in the after- 
noon, the trouble seemed to decrease ; and, at night, 
there was a brilliant aurora prevailing over the eastern 
half of North America, the Atlantic, and northwestern 
Europe: and all telegraphic service was interrupted. 
Cables to Europe, and w T ires to Chicago, could not be 
worked ; annunciators in telephone offices dropped ; 
the switch-board in Albany, N. Y., was ignited; the 
switch-board and wires at Chicago were burned ; and an 
incandescent lamp was illuminated at St. Paul, Minn. 
A message was sent from Bangor, Me., to North Sid- 
ney, C. B., 710 miles, by the earth current alone, with- 
out the batteries ; the current being as strong as that 



THE AURORA. 205 

from 100 cells. And the short line from Boston to 
Declham, ten miles, showed the disturbing influence as 
much as the longer lines." 

In these observations, as in those cited in Chapter XII, 
it has been found that whenever the earth connection is 
severed, at either end of the line, the current immedi- 
ately ceases ; proving it to be an earth current, and 
not a current in the atmosphere. 

The increased intensity of current, on lines having a 
southeast and northwest direction, noticed during the 
aurora of Feb. 4, 1872, is confirmatory evidence of the 
existence of resultant currents, as explained on page 
202. 

The hours at which maximum and minimum effects 
were observed, during the aurora of Nov. 17, 1882, 
correspond exactly to the hours of maxima and minima 
potential, and current intensity, already cited. A max- 
imum having occurred during the three morning hours, 
beginning just before daylight; a minimum late in the 
afternoon, and a maximum again after sunset. 

Another cause of the aurora is found in the move- 
ment of warm air from the torrid to the frigid zones, 
and of cold air, at a lower altitude, from the frigid zones 
to the torrid. The meeting and intermingling of these 
opposite currents, at different temperatures, must give 
rise to strong electric action in the atmosphere, similar 
to that already described as taking place in the earth, 
and coincident with it. And this action must occur in 
the stratum next the earth, far below that assigned to 
the aurora; its intensity increasing with the density of 
the atmosphere, and hence being greatest at the earth's 
surface. 

This becomes evident, when we consider, that the 



206 ELEMENTS OF STATIC ELECTRICITY. 

greater part of the mass of the atmosphere lies near the 
earth's surface ; being included, probably, within the 
first nine miles ; while the auroral stratum is supposed 
to have an altitude of sixty-nine miles. Hence this 
atmospheric electric action would be supplementary to 
that of the earth, already described; and would have 
an east and west as well as a north and south direction, 
as described on page 182. 

The influence of the sun and moon, already referred 
to, must intensify the effects produced by other causes : 
so that we should expect to find maximum and min- 
imum auroral effects, corresponding to an increase or 
decrease of intensity, in solar or lunar influence. Ob- 
servation has shown, that such an auroral maximum 
occurs, during the recurrence, once in eleven years, of 
the period of the maximum solar disturbance; that 
auroras are then more frequent and brilliant than at 
other times : and we may reasonably infer, that future 
observation will show the existence of electric maxima 
and minima, analogous to the tides, and auroral effects 
corresponding to them. 



CHAPTER XIV. 
The Electricity of the Earth and Atmosphere. 



Lightning and Thunder. 

Formation of Thunder Clouds. — Our investiga- 
tion of this subject thus far has been confined chiefly 
to the electricity of the earth and its inductive effect 
on the atmosphere ; we are now to investigate the elec- 
tricity of the atmosphere and its inductive effect on the 
earth. 

We have seen, in the Topler machine, how electric- 
ity is generated by the mutual friction and induction of 
insulated conductors, put in motion by mechanical force ; 
and collected in accumulators which acquire different 
potentials, and between which a discharge finally takes 
place, attended with a flash and report. Something 
analogous to this occurs in the atmosphere. The clouds 
are large conductors, insulated in the air, moved by the 
winds, acting inductively on each other and on the 
earth, and, in other respects, fulfilling the same condi- 
tions found in the machine. 

As the vapor forming these clouds rises from the earth, 
it must have, when generated, the same electric potential 
as that part of the earth from which it rises, and hence 
the same difference of potential which has been shown 
to exist in different parts of the earth's surface. 

The air laden with this rising vapor, moving along in 



208 ELEMENTS OF STATIC ELECTRICITY. 

currents, and brought into contact with elevated parts of 
the surface, and with trees, buildings, and other elevated 
objects, must generate electricity by friction, much in the 
same way as the carriers on the revolving plate of the 
machine. And, as the vapor forms into clouds, they be- 
come the accumulators of this electricity, in the same way 
that it is accumulated by the plates and Leyden jars of 
the machine. And this concentration of electricity in 
the clouds raises their electric potential; and makes 
them the nuclei to which the rising vapor is attracted in 
consequence of its lower potential. 

Each infinitesimal drop of vapor is a sphere with its 
electric charge on the surface ; and as these drops 
coalesce, and form larger ones in the cloud, the charge 
on each new drop accumulates on the surface ; and as the 
increase of volume is greatly in excess of the increase 
of surface, the electric surface density must increase 
in nearly the same ratio ; the volume representing 
electric quantity, which is thus condensed on a reduced 
surface, producing a corresponding increase of intensity. 

Thus as a large body of invisible vapor forms first 
into light fleecy clouds ; and these collect into denser 
masses; there is a constant reduction of volume, and 
increase of electric intensity; till the fully formed 
thunder cloud is the result. 

Discharge Between Clouds. — Two or more such 
clouds, formed in different localities, often many miles 
apart, and electrified in this manner, must, almost inevi- 
tably, be at different electric potentials. And when car- 
ried towards each other by opposite atmospheric currents, 
at different altitudes, and brought within the sphere of 
mutual electric influence, strong inductive effects are 
produced ; their approach is accelerated by attraction, 



LIGHTNING AND THUNDER. 209 

and, when brought within proper distance, a discharge 
takes place from the cloud of higher to that of lower 
potential : just as a similar discharge takes place be- 
tween the sliding electrodes of the machine : and the 
result is chain lightning, of which the spark of the ma- 
chine is an exact type. 

The distance, through which this discharge takes 
place, depends on the quantity and intensity of the 
charge, and the difference of potential between the 
clouds. It may be any distance, from a few yards to 
several miles. Observation on discharges between 
clouds overhanging fixed localities, as two mountain 
peaks, shows that they are sometimes from three to five 
miles or more in length. 

We have seen how sparks, eight to ten inches in 
length, are produced by the machine ; and have tested 
their energy. If we compare such a discharge to that 
produced between two clouds, whose magnitude and 
potential, as compared with those of the machine, are 
almost infinite, we can form some adequate conception 
of the enormous energy of the lightning. 

When the line of discharge is concealed by inter- 
vening clouds, and we see only the illumination result- 
ing from it, the phenomenon is known as sheet light- 
ning. We have the same result, when the spark from 
the machine, occurring in a dark room, is concealed. 
Hence, we may reasonably infer, that the discharge be- 
tween the clouds, like that between the electrodes of the 
machine, would always present the appearance of chain 
lightning, if the line of discharge were always visible. 

The contorte'd and bifurcated discharges, known as 
zigzag lightning, and forked lightning, like similar dis- 
charges in the machine, are doubtless due to differences 



210 ELEMENTS OF STATIC ELECTRICITY. 

of resistance in the air, to the induction of surrounding 
clouds, and to the mutual repulsion of the molecules of 
air and vapor within the line of discharge ; which, be- 
ing electrified to the same potential, tend to separate and 
form resultant lines, under the influence of forces act- 
ing at right angles to each other. 

Observation shows, that there is usually a succession 
of discharges between the two clouds, similar to the 
repeated discharges from a Holtz machine: in which, 
after the initial charge, electricity is generated by in- 
duction alone. This action begins when the edges 
of the two clouds, at different altitudes, approach 
within discharging distance, and come into vertical 
line ; and the effect of induction is to accumulate 
the electricity of the cloud of higher potential at 
the end nearest to the other cloud, while the elec- 
tricity of the latter is repelled to the remote end ; 
just as a similar effect is produced by the mutual 
approach of two differently charged conducting plates 
or cylinders ; the difference of potential between the 
adjacent parts being thus greatly increased. 

The discharge produces a momentary equilibrium, 
which is again disturbed by induction, as larger areas 
of the two clouds approach more closely: the residual 
becoming the initial for a new charge, further conden- 
sation taking place, and fresh supplies of electricity flow- 
ing in from the surrounding atmosphere. In this way 
the series of discharges continues, till the clouds unite, 
and complete equilibrium takes place. 

When several such clouds, at different potentials and 
different altitudes, collect in each other's vicinity ; as 
is usually the case in a thunder storm of much magni- 
tude ; the mutual inductive effect is greatly intensified. 



LIGHTNING AND THUNDER. 211 

Suppose three clouds, arranged in a series, end to 
end, and so graduated as to potential, that the central 
cloud is at a mean between the other two. Let a dis- 
charge take place from the cloud of highest potential 
to the central one ; a second discharge must quickly 
follow, from the central cloud to the one of lowest po- 
tential: since the first discharge has greatly increased 
their difference of potential. This second discharge 
would renew the difference of potential between the 
first and central clouds, and prepare the way for another 
series of similar discharges. 

The most careless observer cannot fail to have noticed 
such series of discharges, following each other in 
rapid succession, in different parts of the sky, during a 
violent thunder storm. 

Observation also shows, that during a thunder show- 
er, there is always an increase of rain-fall, and an en- 
largement of the drops, within a few seconds after each 
electric discharge ; the time being just sufficient for 
the rain to descend, if it left the cloud at the moment 
of the discharge. From which we may infer, that con- 
densation is a result of the discharge ; that, in the mo- 
mentary equilibrium which follows it, the small drops, 
which were before kept apart by mutual repulsion, 
from being highly charged and at the same potential, 
now coalesce, and form the large drops ; which, being 
too heavy to be sustained in the atmosphere, fall. 

Thunder — As the spark from the machine is the 
type of lightning, so the snap represents thunder; 
which is undoubtedly due to the same cause— the sud- 
den and intense vibratory motion of the air, in the line 
of discharge, producing violent undulations in the sur- 
rounding air. A cause which will appear sufficiently 



212 ELEMENTS OF STATIC ELECTRICITY. 

adequate, when we consider the results which must fol- 
low from the rush of the enormous energy of a thunder 
cloud, along a line, perhaps five miles in length, in an 
infinitesimal fraction of a second. 

And here, as in the case of the spark, it is quite un- 
necessary to suppose the passage of any material sub- 
stance through the air, producing partial vacuum and 
collapse, or the occurrence of anything in the nature of 
an explosion, producing similar results. It is more in 
accordance with the known laws of electric movement, 
to suppose that the electric energy has used the air as 
the medium in which to travel; and thus produced the 
vibratory motion. 

Common observation shows, that in explosions where 
the expenditure of energy must often be far less than 
in the electric discharge between clouds, the vacuum 
and collapse shatter window-glass in the vicinity; while 
the heaviest thunder produces only a slight tremor in 
adjacent buildings ; proving that such vacuum and col- 
lapse cannot result from an electric discharge. 

The succession of reports accompanied by a continu- 
ous rumble, heard so frequently during a thunder storm, 
has been considered, by some observers, as a series of 
echoes from a single report; and by others, as a num- 
ber of separate reports, from discharges occurring si- 
multaneously, at different distances from the observer, 
and heard in the order of their distance. 

An echo requires the intervention of an extended 
surface, as a Avail or its equivalent; and observation 
shows, that the under surface of a dense thunder cloud 
is of this character, being remarkably uniform, though 
its upper surface may be quite the reverse : and it is also 
evident, that this under surface, resting on tho denser 



LIGHTNING AND THUNDER. 213 

strata of air, and sustaining the weight of the mass of 
air and vapor above, must have greater density than 
the upper surface. Hence we may reasonably infer, 
that this surface, and that of the earth below it, fulfill 
the conditions necessary for a series of echoes. 

The hypothesis of simultaneous discharges, at differ- 
ent distances, may also be true in certain instances : as 
it is quite possible that such simultaneous discharges 
frequently occur. But the succession of reports, often 
following each other with marked regularity, and steadi- 
ly decreasing in volume and intensity, is not fully ex- 
plained by this hypothesis, while it is entirely in ac- 
cordance with the character of a series of echoes. 

The re-adjustment of electric energy between differ- 
ent parts of a large cloud, which must follow the pri- 
mary discharge, gives rise to numerous minor discharges ; 
whose sound, mingling with that from the larger air 
waves, causes the rumble; analogous to the crackling 
sound from similar minor discharges in the machine. A 
premonitory rumble, from a similar cause, often precedes 
the heavier discharge ; just as the crackling precedes 
the discharge of the machine. 

If the cloud were a perfectly homogeneous conductor, 
like a metal cylinder, this could not occur. But as it 
is a mass of vapor, composed of drops insulated from 
each other by air spaces, each particular drop having its 
own electric charge ; and different parts of the cloud 
having different densities, and hence differing in con- 
ductivity and resistance; and condensation, with increase 
of potential, following the discharge, as already shown, 
such minor discharges, with the accompanying roar and 
rumble, are inevitable. Also the development of the 
residual, after the primary discharge, which, in a large 



214 ELEMENTS OF STATIC ELECTRICITY. 

cloud, must in itself have great energy, greatly intensi- 
fies these effects. 

Discharge from the Clouds to the Earth. — 
We have already seen that the potential of the atmos- 
phere, and hence of the clouds, is almost invariably 
positive with reference to that of the earth. Hence the 
earth's surface under a thunder cloud, and all objects 
connected with it, become negatively electrified by in- 
duction, to the same degree that the cloud is positive ; 
electricity, equal to the charge of the cloud, being re- 
pelled from the earth's surface to its interior. A result of 
this difference of potential is a strong attraction between 
the earth and cloud, by which the cloud is drawn towards 
the earth; and, unless its potential is reduced by discharge 
into another cloud, a discharge to the earth is inevitable, 
whenever, from reduction of distance, the resistance of 
the air becomes less than the electric tension of the cloud. 
When there are two clouds at different altitudes, and 
a discharge takes place from the upper to the lower 
cloud, the difference of potential between the latter and 
the earth, being thus increased, the liability of a dis- 
charge from it to the earth is increased in the same ratio. 
If there are elevated objects, such as trees and build- 
ings, on the surface below, the resistance between them 
and the cloud is less than that of the surrounding flat 
surface; not only on account of reduced distance, but 
also on account of the points and angles which they 
present. Hence, w^e find, that trees, flag-staffs, tele- 
graph poles, church spires, chimneys, and projecting 
corners of roofs are much more frequently struck by 
lightning than flat surfaces. 

Good conductors, such as tin gutters, metal cornices, 
and ornamental iron work, also offer far less resistance 



LIGHTNING AND THUNDER. 215 

than imperfect conductors, like wood, brick, and stone; 
both from their superior conductivity, and their projecting 
edges and points ; and when connected with a building 
and not connected by a metallic conductor with the earth, 
greatly increase the liability of the building, both to re- 
ceive the electric discharge, and to sustain injury from it, 
by making the building its terminus instead of the earth. 

Discharge from the Earth to the Clouds. — 
As already shown, the electricity of a large cloud, like 
that of a cylinder, may be so distributed by the prox- 
imity of one end to another cloud, at a lower potential, 
or to an elevated portion of the earth's surface, that the 
potential of this end shall be higher than that of the 
remote end. The potential of the earth's surface, be- 
neath it, must also be similarly affected by induction, in 
reverse order; being negative where the cloud is positive, 
and positive where the cloud is negative. If, under 
these circumstances, the difference of potential between 
the negative end of the cloud and the earth becomes 
greater than the resistance of the air, a discharge from 
the earth to the cloud must occur; the discharge in this, 
as in all other cases, being from higher to lower potential. 

These conditions are similar to those of the three 
clouds already referred to : so that a discharge from 
the positive end to another cloud, or to the earth, may 
increase the difference of potential between earth and 
cloud at the negative end. 

The resistance of the earth, also, over such an exten- 
sive area, retards the restoration of surface equilibrium 
after the discharge from the positive end; and increases 
the liability of the return discharge from the earth to 
the cloud, in the ratio of this resistance to that of the 
vapor of the cloud. 



216 ELEMENTS OF STATIC ELECTRICITY. 

In this case, as in that of a discharge from the clouds 
to the earth, elevated objects reduce the resistance, es- 
pecially if they are good conductors, or furnished with 
sharp angles or points; and become the electrodes 
through which the discharge takes place. 

Lightning Hods. — Franklin first proposed the 
lightning rod. The identity of lightning and elec- 
tricity, strange to say, was unknown, till, by the erec- 
tion of a metal rod at his suggestion, and subsequently 
by his well known kite experiment, sparks were drawn 
from the cloud, Leyclen jars charged, and various similar 
laboratory experiments, previously known to electric 
science, performed by means of atmospheric electricity. 

The first lightning rod was erected, May 10, 1752, a 
month previous to the kite experiment, by M. Dalibard, 
in France, according to the plan proposed by Franklin 
for testing the identity of lightning and electrichty : 
and sparks similar to those from the electric machine 
were drawn from it. 

The identity of lightning and electricity having been 
established, Franklin showed how the rod could be used 
as a means of protecting buildings. The result is the 
lightning rod, as we now have it, in its numerous forms. 
And though ignorance, greed, and dishonesty have cast 
their shadow upon it, yet thousands of well con- 
structed rods, standing as the silent guardians of life 
and property, sufficiently attest its value. 

The proper construction of lightning rods was re- 
cently investigated by a conference of leading English 
scientists, specially appointed for that purpose : among 
whom were several eminent electricians. And, after 
three years of thorough investigation, during which 
practical information was collected from all parts of 



LIGHTNING AND THUNDER. 217 

the world, a code of rules for the construction and 
erection of lightning rods, or conductors, was adopted 
December 14, 1881; which is substantially as follows: — 

Rules for the Construction and Erection of 
Lightning Conductors. 

Points and Upper Terminals. — As the point of 
the upper terminal, from its peculiarly exposed position, 
is liable to be fused by a heavy charge, it should not 
be sharper than a cone whose height is equal to the 
radius of its base. But, to secure the peculiar advan- 
tages derived from sharp points, three or four such 
points made of copper, each about six inches long, 
should be attached to a copper ring ; which should be 
screwed or soldered to the terminal, about twelve inches 
below its highest point. And all points should be so 
platinized, gilded, or nickel-plated, as to resist oxidation. 

The number of terminals required, their height above 
the building, and the number of conductors connected 
with them, depends on the size and style of the build- 
ing, and the conductivity of the material of which it is 
constructed. 

All elevated parts, such as turrets and spires, should 
be protected by terminals: and especially chimneys, 
whose liability to receive a discharge is greatly increased 
by the heated air and soot. 

Factory chimneys should have a copper band round the 
top; with stout, sharp, copper points, each about twelve 
inches long, projecting from it at intervals of two or three 
feet, and specially guarded against oxidation. And the 
conductor, attached to this band, should be attached to 
all bands and metallic masses in or near the chimney. 

Space Protected.— No definite rule can be given 



218 ELEMENTS OF STATIC ELECTRICITY. 

as to the space protected by a conductor ; as opinion 
and practice vary in regard to it: but there is no well 
authenticated instance of a building furnished with a 
properly constructed conductor, having been injured 
by lightning within a conical space, having the point 
of the upper terminal for its apex, and the radius of 
whose base equaled the height of the conductor. 

Attachment to Building. — The evidence asrainst 
the use of glass or other material, in order to insulate 
the conductor, is overwhelming; and insulation may 
be regarded as unnecessary and mischievous. The 
attachment to the building should be made with metal 
fastenings; which should be of the same metal as the 
conductor itself, to prevent corrosion from galvanic 
action. They should be of adequate strength: and 
each should support its proper proportion of the weight. 
They should not compress or distort the conductor; and 
should allow free play for its expansion and contraction. 

As far as practicable, it is desirable that conductors 
be connected with extensive masses of metal belonging 
to the building, both internal and external; except 
soft metal pipes, which, from low conductivity for heat 
and electricity, are liable to fusion. Gas-pipes, es- 
pecially, should not be so connected on account of 
liability to ignition of the gas by an electric spark, 
resulting from fusion of the pipe, or from bad joints : 
but the inlet and outlet pipes of large gas meters should 
always be electrically connected with each other, as a 
protection against such accidents from the electric 
resistance of joints; which is sometimes greatly in- 
creased by india-rubber packing. 

Church bells, inside well protected steeples, need 
not be connected with the conductor. 



LIGHTNING AND THUNDER. 219 

Ornamental Iron Woek. — All vanes, finials, 
ridge iron work, and similar ornamental metal work, 
should be connected with the conductor: and it is not 
absolutely necessary to use any other point than that 
afforded by such ornamental work ; provided the con- 
nection be perfect, and the mass of iron considerable. 
As, however, there is risk of derangement through re- 
pairs, it is safer to have an independent upper terminal. 

Material for Conductor. — The best material for 
a conductor is copper ; its weight not less than six 
ounces per foot run ; and its conductivity not less than 
ninety per cent, of that of pure copper. It may be 
used either in the form of tape, or of wire cable, in 
which no wire should be less than ISTo. 12 B. W. G. 
Iron may be used, but its weight should not be less 
than 2i pounds per foot run. And all iron conductors, 
whether galvanized or not, should be painted, as a 
protection against oxidation. Copper conductors may be 
painted or not according to architectural requirements. 

Form of Conductor. — The form of the conductor 
does not seriously affect its conductivity : and great ex- 
tent of surface in proportion to mass is not essential: but 
sectional area of mass is highty essential,and should al ways 
be sufficient to carry the heaviest charge without dan- 
ger of fusion of the conductor, or division of the current. 

The rod is desirable for long upper terminals, on 
account of its rigidity ; but the necessity of frequent 
joints, and the difficulty of avoiding disfigurement of 
the building, are serious objections to its use for the 
body of the conductor. 

Tubes are liable to the same objections: their larger 
diameter, and the collars necessary for their joints, ren- 
dering them more conspicuous and undesirable. 



220 ELEMENTS OF STATIC ELECTRICITY. 

Twisted wire cables have the advantage of compara- 
tive freedom from joints ; but their interstices afford a 
lodgment for smoke, dirt, and water; especially if 
small wires are used: which are also less capable of 
resisting oxidation than large wires. 

Tape has the special advantages of requiring but few 
joints; of their being easily made, where necessary; 
and of being flat and flexible, so that it can be adapted 
to the outlines of a building, or countersunk in it and 
painted over, so as not to be conspicuous. 

Conductors should not be bent abruptly round sharp 
corners : and in no case should the length of conductor 
between the two points of a bend be more than one- 
half greater than the straight line joining them. When 
practicable, the conductor may pass straight through a 
projection ; the hole being made large enough to allow 
it to pass freely, without compression. 

The reasons for these precautions are found in the 
liability to discharge from a sharp angle, .or across a 
short space in a bend. 

Joints. — The most fruitful source of danger in con- 
ductors is from bad joints. Screwed, scarfed, or riveted 
joints, however well made, are certain to rust and cor- 
rode in time ; introducing nodes of resistance, at which 
the electric charge is liable either to fuse the conductor, 
or to leave it and enter the building. 

No joint is electrically perfect that is not metallically 
continuous, and as absolutely free from resistance as 
any other part of the conductor: and careful soldering, 
in addition to the screwing, scarfing, or riveting, is the 
only certain means of securing this, which has borne 
the test of experience. 

Earth Connection. — A good earth connection, for 



LIGHTNING AND THUNDER. 221 

the lower terminal, is of the utmost importance ; and 
in a majority of cases of injury to buildings from badly 
constructed conductors, such injury is traceable to 
imperfect earth terminals-. 

The terminal should connect with damp earth, at a 
sufficient depth below the surface, to insure permanent 
dampness, and hence permanent conductivity. And, 
to render this connection more complete, it should 
bifurcate below the surface ; and be connected by sol- 
dering, with a mass of metal, buried in the earth. The 
hole, in which this mass is buried, should be filled to the 
surface with cinders or coke, to facilitate the percolation 
of water; and any available drainage of pure water, 
from rain water pipes or otherwise, connected with it. 

The metal mass may be of copper or galvanized iron, 
having about eighteen square feet of surface. And 
where permanently damp earth is not available, it 
should consist of three or four hundred pounds of iron. 

Where the use of large iron water or gas mains is 
available, a connection by a copper strip, can be made 
with them ; no risk being incurred by such connection, 
as in the case of internal supply pipes. 

Inspect cox. — Periodical inspection, and careful elec- 
tric testing, are requisite to maintain the system in 
efficient order; as points may corrode or become fused, 
joints become electrically imperfect, connections be- 
come severed above or below ground, or other im- 
perfections occur, from alterations in the building, and 
the carelessness or ignorance of occupants or workmen. 



The author has, on his house, a copper tape conductor, 
constructed in accordance with these principles, and 
erected twenty-three years ago ; and neither the house, 



222 ELEMENTS OF STATIC ELECTRICITY. 

nor the conductor, has ever received the slightest injury 
from lightning; while numerous instances of damage 
to buildings and conductors havfi occurred in the vicin- 
ity. Which, considering the length of time, the 
exposed position, and the repeated thunder storms of 
great severity, which have occurred, is strong negative 
evidence of the value of the conductor, and the correct- 
ness of the rules here given. 

Silext Discharge. — The protection afforded by a 
lightning conductor does not consist, so much, in its 
being the avenue by which a destructive discharge may 
pass harmlessly between the earth and cloud ; as in 
preventing its occurrence, by a gradual, silent discharge 
through ■ the points of the conductor; by which the 
accumulated energy is reduced, before it can acquire 
sufficient tension to overcome the resistance of the air, 
and produce a full, sudden, disruptive discharge. 

This is strikingly illustrated by the gradual, silent 
discharge of a large, powerfully charged Leyden bat- 
tery, through the point of a cambric needle ; and is 
confirmed by the brush discharge, often observed, 
during thunder storms, on the points of lightning 
conductors, and on the tips of the masts and yard-arms 
of ships. 

As a building must be regarded, electrically, as an 
elevated part of the earth's surface, the importance of 
as perfect an electric connection between it and the 
conductor, as practicable, is apparent, in order to 
secure the full benefit of protection in the manner de- 
scribed; which is impaired by the resistance caused by 
the use of insulators. 

It is also apparent, that the conductor affords equal 
protection whether the discharge is from the cloud to 



LIGHTNING AND THUNDER. 223 

the earth, or from the earth to the cloud ; as in either 
case, the discharge will follow the path of least resist- 
ance ; which is always through the conductor, when 
properly constructed. 

Heat Lightning. — The phenomenon, known as 
heat lightning, is probably nothing more than the or- 
dinary electric discharge from clouds invisible to the 
observer, and so distant that the thunder is inaudible. 
Such lightning is generally observed at night, near the 
horizon ; and close observation will show, either the 
existence of clouds, indistinctly visible in the darkness, 
or the probability of the discharge occurring from 
clouds below the horizon. 

Its existence, independent of clouds, is claimed from 
the fact, that it has been observed when no thunder 
storm had occurred within a radius of one hundred 
miles. But, not only lightning, but clouds are often 
visible at greater distances. On the level surface 
round Chicago, the author has frequently observed 
heavy thunder storms, eighty miles distant, as shown 
by subsequent reports, when both clouds and lightning 
were distinctly visible, though the thunder was not 
audible. 

Tornadoes. — As an electric origin has been claimed 
for tornadoes, it is proper to remark, in conclusion, 
that recent investigation has demonstrated that they 
are chiefly due to currents of air, generated by differ- 
ences of atmospheric temperature and pressure, and 
modified by other causes: and while electricity may 
intensify their force, it cannot be considered as their 
primary cause. 



224 elements of static electricity. 

Note referred to ox Page 118. 

The brush from K makes its appearance first, and 
increases in length till the brush from V appears; after 
which it decreases in the same ratio as the brush from 
V increases, till the discharge occurs, when both dis- 
appear. This is sufficiently explained by increase and 
decrease of difference of potential at different points. 
As the potential of the revolving plate A increases, the 
difference of potential between the inside coating of the 
jar (7, and that part of A which receives the charge 
from it through the comb K, decreases, as indicated by 
the decrease in brush-length, till the potential of both 
is the same, when the brush disappears. 

In like manner the potential of that part of the plate 
A, passing the comb i, continues to increase till it 
equals the potential of the inside coating of the jar D; 
and this charged surface, passing on to the comb H, the 
surplus of charge which D, from increase of potential 
rejects, escapes through H to the comb V, and from V 
to that part of the plate A between V and if, as indi- 
cated by the increase of brush-length from V. 

This process is greatly intensified by the inductive 
effect of the high potential of the lower part of inductor 
J 7 , and low potential of the upper part of inductor X, 
by which electricity is repelled from the corresponding 
lower part of the plate A to its corresponding upper 
part. 



INDEX. 



Absolute Electrometer, Thomson's, 161-169. 

Accumulators, 72-91. 

Amber, 1. 

Atmosphere, the, as a Leyden jar, 180, 181. 

Atmospheric potential, 177-1S0. 

strata, difference of potential between, 
179, 180. 

currents, 181-183. 
Attraction and repulsion, 1-4, 15, 40-42. 
Aurora, the, 190-206. 

, height of the, 198, 199. 

, geographical position of the, 199-201. 

, causes of the, 201-206. 

, tubes, 146, 147. 
Auroral arches, coronas, and streamers, 191- 
195. 

movement, curtain formation, 195-197. 

bands, 198. 



B 

Bag experiment, 60. 
Balanced rod, the, 2. 
Bath, electric, 142, 143. 
Battery, the Leyden. 79, 80. 
Bells, electric, 102, 103, 125, 126. 
Brush discharge, 117, 118, 134, 137. 



Charge defined, 22. 

.multiplication of, in Topler machine, 
121, 122. 

, variation of, 67. 
Charged surfaces, formulas for, 167. 
Chime, electric, for frictional machine, 102, 
103. 

. for Topler machine. 125, 126. 
Condensation, surface, 55-58. 
Condensers, 74. 

15 



Conductivity for heat and electricity com- 
pared, 37, 38. 
Conductors and non-conductors, 4-6. 

, hollow, 58, 59-66. 
Conservation of energy, the, 23-26. 
Convection, 66, 67. 
Cosmic electric influence, 183-186. 
Coulomb's torsion balance, 156-161. 
Currents, atmospheric, 181-183. 

, earth, 1S6-189, 204, 205. 
Cylinder, electrified, 48. 69. 

with points, 70. 



D 

Dielectric denned, 50. 

, required thickness of, 74. 
Disc, electrified, 71. 
Discharge, apparent time of, 126-128. 

, brush, 117. 118, 134, 137. 

between clouds, 20S-211. 

from the clouds to the earth, 214, 215. 

from the earth to the clouds, !J5, 126. 

, disruptive, 88. 

, silent, 89. 

, spontaneous, 88. 

through hook, 81-S4. 
Discharger, 76. 

, universal, 87, 88. 
Dual theory, the, 40-42. 



E 

Earth currents, 1S6-189, 204, 205. 
Ebonite, 1,6, 58, 54. 

Electricity, the nature of, 23-42. 

of the earth and atmosphere, 175-223. 

generated by the friction of metals, 132. 
133. 
Electrics. 4. 
Electric bath, 142,143. 



226 



INDEX. 



Electric movement, 13-16. 

potential, 10-11. 

transmission in vacua, 148-154. 

wind, 104, 105. 143. 
Electrometers, 155-174. 

, attracted disc, 161. 
Electrometer, Thomson's absolute. 161-169. 

,mode of using the absolute, 166-169. 

, Thomson's quadrant, 169-174. 

, mode of using the quadrant, 173, 174. 
Electrophorus, the, 92-96. 
Electroscope, the gold leaf, 16-18. 

, the pith ball. 2 3. 

, charged by induction, 44. 
Energy, the conservation of, 23-26. 

, radiant, 31. 
Ether, 31-33. 
Equipotential. 55. 
Experiments with the Topler machine. 125-145. 



F 



Farad iv's hollow cube, 65. 
Faradic current. 141. 
Figures, Liehtenberg's, 89-91. 
Force, 1. 

, lines of. 55. 
Form, influence of. 67. 
Formulae for charged surfaces, 167. 

, application of, to measurement by elec- 
trometer, 167-109. 
Fracture of Ley den jar, 83, 140. 
Friction, mutual effects of, 18-21. 
Frictional electricity, 8, 9. 

machine, 96-100. 



G 

Gauge, idiostatic, for electrometer, 63. 
Gas lighting, 143-145. 
GeissLr tubes, 147, 148. 
Generators, electric, 92-124. 
Glass for Leyden jars, 77. 

illuminated by electricity, 151. 

, required thickness of, for insulation. 74. 

, specific inductive capacity of, 53, 54. 
Gravity and electiicity compared, 13, 14. 
Gunpowder, method of exploding by elec- 
tricity, 87. 



Heat and electricity compared, 13, 14, 33, 37, 

38. 
Heat, light, and electricity compared, 26-31. 
Heat lightning, 223. 



Heating effects of electricity in high vacua, 

153, 154. 
Hollow conductors, 58-66. 
Hollow cube, Faraday's. 65. 
Holtz machine, the, 108-110, 122-124. 
Holtz and Topler machines compared, 122- 

124. 
Holtz, Dr. W., correspondence with, 123, 124. 
Hydro-electro machine, Armstrong's, 105- 

107. 



Idiostatic gauge for electrometer, 163. 
Image plates, 103, 104. 
Induction. 43-54. 

, theory of, 4S, 49. 

varies inversely as square of distance, 
47. 
Inductive capacity, specific. 51-54. 

influence of dielectric, 49-51. 
Influence machines, 108. 
Insulator defined, 6. 
Intensity, electric, 6-8. 



Jar. the Leyden. 75-91. 

Jar D,in Topler machine, higher potential of, 
139, 140. 



Leyden jar, the, 75-91. 

, charged by cascade, 77-79. 

.discharged through booVc,81-S4. 

, electromotive force of, 77. 

, fractured by overcharge, 88, 140. 

, glass suitable for, 77. 

, Lane's unit, 101, 102. 

, residual charge of, 84, 85. 

, spontaneous discharge of, 88. 

, the atmosphere as a. 180, 181. 

. with movable coatings, 85, 86. 
Leyden battery, the, 79, 80. 

, Tyndall's experience with, 87. 
Liehtenberg's figures, 89-91. 
Light, heat, and electricity compared, 2^31, 
138. 

, polarized and electricity, 28-31, 3(5. 
Lightning and thunder. 207-223. 
Lightning conductors, 216-221. 

, attachment of, 218. 

, earth connection of, 221. 

, form of, 219, 220. 

, inspection of, 221. 



INDEX. 



227 



Lightning conductors, joints of, 220. 

, material for, 219. 

, points for, 217. 

, silent discharge of, 222. 

. space protected by, 217, 218. 

, test of copper tape, 221, 222. 
Lines of force, 55. 



M 

Machine, Armstrong's hydro-electric, 105- 
107. 
described by Xoad, 100. 
, Motional, 96-100. 
, the Holtz, 108-110. 
, the Topler, 110-122. 
Machines compared, Holtz and Topler, 122- 
124. 
, influence, los. 
Measurement of energy, 100-102. 
Medical treatment by electricity, 142, 143. 
Metals electrified by friction, 4, 5, 132. 
Metal screen, inductive action of, 152, 153. 
Mode of action of the frictional machine, 99, 
100. 
of the Holtz machine, 122, 128. 
of the Topler machine, 115-124. 
Multiplication of charge in Topler machine, 
121, 122. 



N 

Nature of electricity, 23-42. 
Negative charge, 22. 

potential, 12, 13, 21. 

sign, 13. 
Non-conductors, 4, 5, 6. 
Non electrics, 4. 



o 



Ozone, generation of, 131. 



Pail experiment, CO-65. 
Pane, the charged, 72-74. 
Plates, image, 103. 104. 
Points, air current from, 104, 105. 

, influence of, C9, 70. 
Polarized light and electricity, 28-31, 36. 
Proof plnne, 58, 59. 
Positive and negative, 12, 13, 21. 

sign, 13. 



Potential, atmospheric, 177-180. 
and earth currents, 175-1S9. 
, el-ctric, 10-22, 
, difference of, 11, 12. 
, difference of, between atmospheric 

strata, 179, 180. 
, diurnal and seasonal variation of, 177- 

179. 
of jar D, in Topler machine. 139, 140. 
, reversal of, in Topler machine, 120, 140. 
, zero, 13, 65, 66. 
Power, transmission of, by static electricity, 
128, 129. 



Quadrant electrometer, Thomson's, 169-174. 
Quantity and intensity, 6-8. 



R 

Radiant energy, 31. 
matter, 154. 

Replenisher for electrometer, 164. 

Repulsion, 1-4, 15, 16, 159. 

Residual charge, 84, 85. 

Reversal of potential in Topler machine, 120, 
140. 

Rotation of Topler machine, direct and re- 
versed, 138, 139. 

Rotary movement in vacua, 149-151. 



s 



Silent discharge, 89. 

Source of electric supply of the Topler ma- 
chine, 129-132. 
Spark, the, its direction, subdivision, and 
color, 133-138. 
, and snap, 39, 40. 
Specific inductive capacity, 51-54. 
Spheres, electrified, 68. C9. 
Spheroid, electrified, 70, 71. 
Spontaneous discharge, 88. 
Static electricity defined, 8, 9. 
Surface condensation, 55-58. 
, thickness of electrified, 66. 
transmission, 58. 



Telegraph lines, observations on, 1S6-18S, 204, 

206 
Tides, electric, 183, 184. 



228 



IXDEX. 



Time of electric discharge, 126-123. 
Thermopile, illustrations from the, 175, 17< 
Thickness of electrified surface, 66. 
Thunder, 211, 214. 

clouds, formation of, 207-208. 
Topler machine, the, 110-122. 

, the four-plate, 114. 

, experiments with, 125-145. 

, mode of action of, 115-124. 
Tornadoes, 223. 
Torsion balance, Coulomb's, 156-161. 

, inaccuracy of the, 159 161. 
Transmission, electric, in vacua, 146-154. 

of power by static electricity, l_s, 129. 

, surface, 58. 
Tubes, Geissler, 147, 148. 
Tube, vacuum, 146, 147. 



U 

Universal discharger, 87, 83. 
Unit jar, Lane's, 101, 102. 



Vacua, electric transmission in, 32, 146-154. 

, electric transmission in low, 146-149. 

, electric transmission in high, 149-154. 

, rotary movement in high, 149-151. 
Vacuum tube, 146, 147. 



w 



Wave theory, the, 31-37. 
Whirl, the electric, 104. 
Wind, electric, 104, 105, 143, 



Zero potential. 13. 65, ( 



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